Huh? Literally the first sentence in that article starts with
> At-will employment is a term used in U.S. labor law for contractual relationships [...]
Emphasis on contractual.
Or am I missing something?
A contract that can be dissolved at will is still a contract. Just like you can cancel your Netflix subscription at will, and it's still a valid legal contract.
https://en.m.wikipedia.org/wiki/At-will_employment