I think the market making profits from Tether have been pretty substantial over the years. They charge very high redemption fees, which means that either they, or some close partner of theirs is the primary liquidity provider. Tether is usually mostly on peg, but it is frequently off by a few basis points, sometimes a few tens of basis points, and occasionally a percent or two. The liquidity at these levels is often quite deep, meaning you can earn fairly substantial profits by buying it up.
It's not just the headline moments when it's off peg, it's the daily liquidity churn of people willing to sell a few basis points below par, because they'd rather get out. Look at this price chart:
https://coinmarketcap.com/currencies/tether/
And look at the daily volume Tether experiences. During 2021 it traded roughly ~$100B in volume per day. One basis point on that is $10M/day. Of course, they're not a counterparty to every Tether trade, not even close, but even a fraction of that is very good money, and remember it was frequently off peg by more than one basis point. Also keep in mind the point of comparison here, which is essentially short term treasury rates over the last several years. I think the MM profit on Tether has certainly been at least close enough to match if not significantly exceed the yield on cash equivalents over Tether's lifetime (until very recently).
Note that Tether is also frequently off peg in the other direction, and they can exploit that too by minting and selling USDT into the market when that occurs. Of course, that has nothing to do with them looking dodgy or not (it's correlated to sharp, unexpected liquidity demands by retail).