It's most likely a standard "at will" employment yes.

They have a contract, but the contract will specify them as "exempt salaried" employees. That's pretty standard across the board in the tech world (as well as many other professional roles).

Exempt salaried employees aren't paid by the hour, and don't have a set number of hours they are expected to work. There is no overtime, but there's also no (contractual) penalty for working fewer than 40 hours in a week.