But they get to buy and sell tethers themselves, right? If a T costs less than a dollar, and Tether's owners know that it's perfectly backed by a liquid dollar, why don't THEY buy it?

If they don't, it suggests it's not, and they actually agree it's worth less than a dollar.

Maybe they are buying them? Just not at enough speed to avoid a decrease in price.

To buy them they would need to take their real USD, probably spread across investment instruments (e.g. in an ideal case where all they say is true, they would sell some government bonds they're holding), and bring it to an exchange where USDT is below 1 dollar. Then on that exchange they would go and buy, and make a small profit.

Based on the fact that they have managed to keep this thing around 1 USD for a long time, they have probably been doing this. But we don't know how fast they can do it (have to be faster than the others selling it), and we don't know how much cash they really have so they may have to stop doing it at some point if enough people try to sell.

There are rumours, for example, that quite some of Tether's reserves are in bonds (treasuries etc.) or commercial paper. There is secondary market for bonds and even commercial paper, but it is rather slow (OTC, not on an exchange), and if you need to sell bonds or paper in distress, guess what sort of price you're going to get...