FTX blew up because they used their own token (FTT) as a collateral, not because they were simple holding FTT. Binance doesn't use BUSD as a collateral, but of course they have a supply of their own token.
I don't get what's the point of holding your own stable coin except to potentially double the money they have?
Lets say they have $100M in cash(from wherever). Now they print 100M BUSD with the earlier $100M backing it up. Now they have $200M that they can borrow against?