Nope. Not a fact at all. Every top university, and lots of non-top ones, has a long list of “billionaires and multi-millionaires” who went there. Because Harvard’s undergraduate college is so much smaller than, say, Berkeley’s or UCLA’s, it’s doubtful that Harvard has “far” more billionaires and multi-millionaires than they do, and it may not even have more at all.
If you limit yourself to billionaires, Harvard may come first, but not by some huge margin. And the chance of becoming a Harvard-alum billionaire without starting with at least tens of millions of inherited wealth is tiny.
EDIT:
Here’s the list of universities with the most undergraduate alumni on the Forbes 400 list for 2016:
University of Pennsylvania – 21
Harvard University – 14
Yale University – 14
Stanford University – 14
University of Southern California – 11
Cornell University – 9
Princeton University – 7
Columbia University – 6
Dartmouth College – 6
University of Michigan – 6