That thread seems to be insinuating that startup executives were offered mortgages in return for keeping their treasury as deposits in SVB. If so that sounds like a massive violation of fiduciary duty. (I don't have any concrete evidence of this and I'm just going by one Twitter thread I've read, so take with a mountain of salt.)
the issue is that startup founders might have a lot of implied wealth based on the equity they hold and money raised but a "mainstream" bank is going to look at that equity, assess it as non-liquid and highly speculative and reject any loan applications
svb was more likely to extend personal loans to startup founders because -- in theory at least -- they better understood startup finance and they were incentivized to provide good service to prospective customers of their more business focused activities
Perhaps some near-fraudulent collusion between SVB, VCs and the executives of the banked startups then ...