Hitler, and even more so Goebbels, did have astrologers. Goebbels had had horoscopes prepared on January 30, 1933 (the day Hitler took power), as well as at other times, in an effort to predict the future of the Third Reich and the outcome of the war. The belief in astrology by some top Nazis played a role in their propaganda and efforts to reassure themselves and the German people that Germany would win the war. Hitler didn't directly govern based on what astrologers told him or his associates, but he and other Nazi leaders did have a mystical faith in astrology.
I've never heard that Hitler was influenced by any astrologer as far as his disposition of forces on D-Day went, and while such a thing may have played a part in his decisions, it's most likely an invention, or at least exaggeration, by the filmmakers. Still, the film didn't have to invent Hitler's and the Nazi hierarchy's interest in astrology, since this was in fact basically correct. In any case, I'm sure no one in the government was consulted or had anything to do with making sure the existence of "Garbo" wasn't revealed. Given the one-dimensional portrayal of Hitler in this movie, depicting him as a clownish nut guided solely by astrological predictions would fit right in with the movie's somewhat silly conceptualization of him.