Tether is backed 1:1 with USD in traditional bank accounts. Some of it may still be seized and held by the US government, not sure if that's been resolved yet.
USDT also trades freely on open markets against USD. If USDT was being printed without USD backing then those markets would decouple.
USDT is another "cryptocurrency". I put that in quotes as they can print it at their leisure. They claim to be backed by USD but have never been audited. The market cap grows by billions a month.
BTC and USDT is a common trading pair on crypto exchanges.
As a result, those behind USDT can print it and buy BTC to manipulate the price.
Don't get me started on exchanges allowing you to trade with yourself. This field is tainted with shady practices.
People are buying and selling tons of BTC with USDT and if it ends up being a fractional reserve and has been printing billions with nothing backing it up, how much of the BTC price might've come out of thin air?
USDT also trades freely on open markets against USD. If USDT was being printed without USD backing then those markets would decouple.