USDT comes in many forms (technical side). You literally can trade USDT as one of the only major stablecoins over bitcoin+lightning network but that is not the entirety of all USDT in circulation. If you look into how this works (because it is fully decentralized running over bitcoin, not some proprietary blockchain) you will find out you can exactly get the market cap, which bitcoin wallet hold how many coins etc. Now lightning distorts that a bit, but market cap is not affected by that.

But it is in the nature of stablecoins that you cannot check their vault. You have no idea if they actually store the assets they claim to store. And whether they will honor their promise to exchange that asset for their tokens.