One theory is that it was their users. That is, Alameda lost huge amounts taking the other side of trades on FTX, and then FTX stole customer money and gave it to Alameda to compensate
The block chain shows the bitcoin moving from one address to another. That's about it. It doesn't say what was exchanged for it, be that USD, FTT, or any other token, good, or service.
And those addresses are potentially pseudonymous to boot, at least if the BTC is withdrawn. If it ISN'T withdrawn, it's entirely plausible that the transactions took place off chain in the first place -- simply moving numbers around in an internal database while staying right in the same, FTX owned wallet. What you'd call in the trad fi space "holding assets in street name."
Btw, I find this explanation compelling - I'm not challenging it necessarily.