"margin requirements are 50%, five times what they were then"

Oops

FINRA Margin Debt https://share.google/2Y3ZcRNBlnCguTPK7

Today the 50% number is only for Reg-T, and even then the net account requirement (maintenance) is lower. But there's also portfolio margin, which gives you much more leverage, and a lot of people end up using it who wouldn't otherwise because Reg-T doesn't handle offsetting positions (i.e. a hedge counts as more risk, not less, for purposes of buying power).