According to the argument against Tether, your first principal here is wrong. In this argument, Tether mints a USDT and trades with someone who has bitcoin or other crypto and wants a stablecoin. What the trader believes they are getting is a coin which is 1:1 backed by USD. In this situation, it is not.
Tether gets bitcoin. Tether can then print more USDT which they trade for bitcoin at a higher price. They would then have an infinite money cheat for buying crypto assets (as long as people believe a USDT = $1) which enables them to drive the price of crypto upward.
Tether claims that the pattern is "Someone gives us USD, we give them Tether, they trade for whatever they want", this was provably false at some points in time.