Is Himmler's memo about genocide authentic? Significant?

It was well known that Himmler was not a thinker. "Himmler's brain is called Heydrich". In strategic matters he simply used the opinion of several officers of his staff. Before 1943 - that of Heydrich.

I have read that Himmler considered the annihilation of three millions of Soviet captives at the first 4 months of the German-Sovied war as an error. Because most of them were ready to work and many to fight on the German side. Himmler was heartless, but pragmatic. And being not pragmatic he( or Heydrich at his shoulder), obviously, considered not German. That was his utmost "rebellion" against Hitler. He did annihilate them, but considered it as ineffective solution.

So, it is absolutely unimportant what opinion did or did not have Himmler. He always only did what was ordered. He was a very good organizer and organized what was said to him. To work out some strategic opinion - you are welcome, he organized a good staff and expert groups. To waste millions of people? - you are welcome, he organized camps and staff them with people of other sort.

It was important what Speer or Goering thought - for they argued with Hitler. Goering even said: "In Luftwaffe(Airforces) it is me who decides, who is Jew and who is not." Speer had sabotaged some Hitler's orders. Even opinion of some people in lower positions, such as Heydrich or even Guderian, had value, for they stood for their ideas and they proposed them. But Himmler or Goebbels simply silently did what was ordered or said or merely wished by Hitler.

Some expert group in SS made some memorandum - why not?

Edit to add: Felix points out in his answer that the Himmler memo did not refer to Jews, but to slavs inhabiting an area allready free of Jews. While I do believe that the Nazis radicalized themselves in the timeframe, it does not follow at all that Himmler was not as genocidal in 1940 as he would get, so the whole point of this answer breaks down. I still leave it as I believe the context is useful.

I admit to lack in depth knowledge of Himmler, his relation to Hitler and the Nazi bureaucracy. I'll offer an interpretation by situating the memo in time.

The memo was written in May, 1940. Up until then the Nazis had invaded Poland, maybe 65.000 civilians had been murdered by Einsatzgruppen until the end 1939. Additionally, the Einsatzgruppen - under the command of Heydrich, Himmlers subordinate - drove Jews into the first Ghettos. Most Ghettos, and the largest, where only established in 1940 and 1941.

Meanwhile, on the territory of the Reich, the systematic murder of disabled persons with gas had started in January 1940. Aly, Browning and other historians think that the experience of these "euthanasia" murders where important stepping stones to the Shoa: The Nazis learned that they could murder many adults "industrially", that persons from civil society - doctors, police, judges - would go along. This experience partially formed the decisions made in 1941 that ultimately led to the Shoa - but only four months after the start of the murders, it was far from sure fi the program would be a "success"

From this, I would say that Himmler's memo means that in early summer 1940, he thought that murdering every Jew by mass shooting would not be feasible. The Nazis had not yet made the experiences of the "euthanasia"-murders, that would embolden then later.

An additional factor that may have changed Himmler's mind would be the impossibility of deportation to Madagascar, that was evident in 1941. In 1941, the Generalplan Ost was formalized, that called for "emptying" by murder (through starvation and death camps) of large parts of the occupied Soviet Union.

Authenticity

The memorandum, titled "Reflection on the Treatment of Peoples of Alien Races in the East", is certainly genuine.

Though if I understand correctly, what we have is not the memorandum (said to be 6 pages long) itself, but rather Himmler's own summary of it and his further notes to self on dissemination.

These notes were entered as Document No-1880, Prosecution Exhibit 1314 at the Nurenberg military tribunal.

There is a full English translation available on a number of websites. E.g. here.

Apparently, Generalplan Ost was based on the ideas raised by Himmler in his memorandum.

Significance

1. Most emphatically, Himmler (or other top Nazis) was not against mass murder. On the contrary, by the time the memorandum was written, the Einsatzgruppen had already murdered at least 65,000 Jews (as others have noted in their answers/comments here already).
2. However, it does seem to be true that at that stage Himmler (and other top Nazis) did not consider outright mass murder as the only way to "solve" the "Jewish problem". What Himmler writes later on in this document is:

I hope that the concepts of Jews will be completely extinguished through the possibility of a large emigration of all Jews to Africa or some other colony.

Again, others have pointed out that what he had in mind was a ghetto writ large, at best. Recall that the official Nazi line was not that Jews were being murdered but rather "resettled in the East". So the Madagascar project might as well have easily morphed into a huge death camp too had it been implemented.

3. When context is considered, it becomes clear that the quote about un-German methods was not even referring to Jews. This is the full context:

For the non-German population of the East there must be no higher school than the four-grade elementary school. [...] Apart from this school there are to be no schools at all in the East.

Parents, who from the beginning want to give their children better schooling in the elementary school as well as later on in a higher school, must take an application to the Higher SS and Police Leaders. [...]

If we acknowledge such a child to be as of our blood, the parents will be notified that the child will be sent to a school in Germany and that it will permanently remain in Germany.

Cruel and tragic as every individual case may be, this method is still the mildest and best one if, out of inner conviction, one rejects as un-German and impossible the Bolshevist method of physical extermination of a people.

As explained before, these pipe dreams of Himmler are referring to an East already cleared of Jews. So, he is holding forth about the various Slav peoples here and comes to the conclusion that mass-murdering them might not be practical after all, and perhaps not very sporting. Even such a "courtesy" is not extended to the Jews.