The argument isnt that there are no legitimate inflows of other stablecoins (or traditional fiat currency) into bitcoin prices. The problem is that Tether is ~80% of inflows [1], and is dubiously backed at best. If 80% of the price support disappears, prices will fall, and legitimate buy side interest from other stablecoins or fiat will almost certainly shrink as well.

[1] https://twitter.com/JacobOracle/status/1346133087877537792/p...