Not just being partner, but what sounds like some extraordinary generosity by Paul Graham, new in the WSJ article cited by OP:

Hydrazine bought out a portion of startup shares owned by Graham, a transaction that gave Altman stakes in some of the hottest companies backed by Y Combinator. The sale hasn’t been previously reported.

https://archive.is/x2hx4#selection-2672.0-2672.1

Depending on what companies and how much, that could have been an extraordinary windfall.