If the lateness is the result of a delayed inbound, the GA has access to this information, and I would expect some more leniency since they can know that the passenger is "on the way."
For self-inflicted lateness (getting to the airport too late, meals (as in the OP), etc.) I would expect less leniency and a sharper cutoff.
Cheers.
I don't think that's what Ambraciot was asking. The question, as I understood it, is -- what happens if, say, your 4pm flight gets delayed to 4:30pm and you don't show up to the gate until, say, 4:10pm. Could they have given your seat away? I think the answer is no, but it seems to be one of those things that isn't covered by a hard-and-fast policy. Sometimes they'll post a (long) delay but the GA will tell people not to leave the gate area since they may get permission to board/depart earlier (usually the case with ATC delays). But what's the "official" policy here?