I agree that they share naturalistic assumptions, but humanism ends up being more dualistic than monistic, I think. Humanists are effectively anthropocentric, so humans replace gods as the measure of all things, as Protagoras put it. Buddhists seem at first much humbler and less Luciferian or Promethean, in rejecting that kind of hubris. But then we face the question I'm raising: Why are Buddhists so interested in eliminating suffering or in being happy in the sense of being tranquil and unperturbable? The answer seems to me to take the Buddhist into humanism.