I don't think anyone knows for sure. There is a decent chance that Tether is (or was before the crash) decently backed by "stuff" - just not with what it claims to be backed by. I'd imagine other crypto holdings were a significant part of it. In other words, it's not really a stablecoin, but some other complex derivative instrument. So it depends on how you define scam. It's certainly false advertising and has always been.