I'm not making these claims you seem to think I am.
I am 100%, absolutely, positively not saying 1:1 backing is required for a currency peg.
I am saying that in the case of tether, that is the explicit claim they are making about their cryptocurrency. They are claiming that each and every one is backed by a dollar in their account. This is their value proposition, this is what you are buying when you buy tether. An open, audited, 1:1 backed US dollar token * .
Please go and read their homepage if you have some sort of issue with this, it's right there at https://tether.to/
The idea is very much that you can park money in tether and it's not vulnerable to a run on the currency and it cannot drop below a dollar, because each one has a corresponding dollar in an account.
(* unfortunately they are not very open and have never been audited)