One would assume Softbank made sure that the debt was collateralized with something other than Better.com shares. I'm sure a good chunk of that 750M is collateralized with better.com shares but I'd be willing to bet they made him put some other stock/investments on the line.
If softbank believes in Better, but not necessarily in Garg, then having him personally guarantee the loan against his own shares is an easy way to push him out cleanly if things go south.
But I'd assume a huge amount of that net worth is due to Better.com's valuation. Valuation is, of course, just the amount the most recent investors paid for their shares, times the total number of shares. And those most recent investors include Softbank, who also gave the founder the loan.
Net worth is an abstraction that's generally useful to reason about outcomes, but it's a leaky abstraction.