Clearly, someone, presumably Bitfinex, is propping up the market. That's what they are supposed to be doing. The question is whether they have the financial strength and will to do it in times of stress.

Historically, price pegs like this break at some point.

Think of Tether as a trading convenience. It's needed only because Bitcoin "exchanges" are so difficult about wire transferring money out. (With real stockbrokers, cash can be wire transferred out within hours with no nonsense. A firm that can't do that goes bankrupt within days.) Don't keep money in Tether for long.