That’s for “at will” employment in the US. Other countries actually have rules you need to follow, both ways. The employer needs to give you a grace period, and you need to as well, under risk of penalties or invalidation of unemployment benefits.
So no, it isn’t just a “I quit” letter.
Another primo example of American exceptionalism.
See, as opposed to resignation, there are no rules about being allowed to shame idiots online.