keraveltheint, once again thanks for the source. Tanya does indeed say it the way you put it!
Btw I note that at the end of that letter the Baal Hatanya writes that after his petira Moshe Rabbeinu’s neshama went into the 600,000 neshamos of Klaly Yisroel in each generation, something which in another much much much longer thread I argued was what was meant by “ispashtusa deMoshe bechol doro”, That is, the Zohar does not seem to be saying that in each generation there is only one person with the neshomoh of Moshe Rabbeinu, as has been claimed elsewhere.