So my understanding is that alameda needed more capital to fund its operations that had a track record of success, so sbf created ftx and gave customer assets to alameda to invest. So the business thesis here isn’t “use new user money to pay old users” it’s “steal all the users money to invest and hope the investments continue to be profitable.” I guess they started ponzi-ing once alameda went bust but that wasn’t the original plan, although that’s probably true for most ponzi’s.

According to Wikipedia a Ponzi scheme

> is a form of fraud that lures investors and pays profits to earlier investors with funds from more recent investors.

So if they are paying yield while actually losing money and covering it up by recruiting new clients then it's a Ponzi scheme.