So, if I "check in" for my reservation at Jiko and my table isn't yet ready, does that mean I haven't checked in? They find my name, check me off, hand me a little pager thingy, and tell me that my table will be ready soon. Doesn't all of that constitute being "checked in"? When I check in at the airport ticket counter, I don't get to board right then and there. So am I not checked in until I'm actually on the plane? I've shown my ID, they mark me as having arrived, and I'm at the gate waiting until I'm allowed to board. Doesn't all of that constitute being "checked in"?
Checking in means that you've made yourself known to the checker (in the case of a resort, the front desk) and they have noted your appearance on whatever list they check you off on. It doesn't mean that you instantly get a room (or seat or table).
The dictionary definition of "checking in" is
"act or instance of showing one's ticket to or registering with an authorized agent, as in an airport or hotel". It says nothing about taking possession of your room. Checking in tells the resort that you're there, nothing more. While it is common practice to have a room available for you at that time, I've never seen a guarantee of that on the resort documents (at Disney or anywhere else).