This says that he should’ve taken his $1.3B in 1988 (which he earned in business from the original loan/inheritance of under $100M) and invested it and made more.

This is interesting, but also a little beside the point because he still turned millions into billions before then with, presumably, business skill.

I see this stated everywhere. What is the source for this?

My understanding is he got a $1M loan. $1M invested in an index fund for 40 years at approx. 8% annual return gets you... a resounding $24M. If did it on a $40M inheritance, he would have under $1B.

He would need to have invested $100M for 40 years to even get…