That's something different, though. Money market funds are meant to be safe interest bearing investments: they're expected to give a small positive return on investment in normal times, and to be safe enough that people will at least get their original investment back in bad times. That's why it's a big deal when something happens which causes them to return less than was invested by any amount: a safe investment, which gave lower returns in exchange for that supposed safety, wasn't.

The goal of Tether is a little different. One USDT is meant to be worth one USD. It's just as much a problem for the purposes people use it for if USDT is trading above one dollar as below, because they're paying more than its value. That is, the Tether peg is meant to be two-sided, both above and below, and all that's happened is that it's gone from trading at slightly above the nominal value to trading slightly below it. That's not really "breaking the peg" in any meaningful sense.