Sure, I'm not disputing this, just giving one plausible way Tether might be getting real USD.

Oh, they're certainly getting some.

The suspicion is they're siphoning it off to their personal accounts, and that they're claiming to have received way more USD than they actually have so they can issue USDT that aren't actually backed to manipulate the market.

In that case, I'm confused by your question: "where's the USD to buy the Tether coming from?"

You seem to have changed what you're suspicious about.

For clarity, there's a variety of very suspicious things about Tether. While they're clearly receiving some USD, they're printing $100M in Tether every couple days now.

If I claim to have $50k, you'll probably assume it comes from my job. If I claim to have $2B, as Tether does, you'll probably want some proof. Tether's promised regular audits, but they've released none and their supposed auditor has now scrubbed any mention of them off their site.

There's been a fair amount of conspiracy-theory-like thinking around Tether in general, with people making a morass of self-contradictory claims about it and shifting their positions every time someone points out one of the claims doesn't stand up.

To be fair, tether really is quite shady. I think the real truth is probably somewhere in between.