I did meetings and events for years as well.
Did I care what the caterers or venue did with the leftovers? Heck no.
The fact that this even came on someone’s radar tells me either they were not done with the platter or, as you indicated,
this was a convenient excuse.
Another person suggested the liability angle, wherein if they got sick from the food and wanted to sue, the company could be exposed (even fighting a nuisance suit is expensive).
It’s interesting that the staff (read secretaries, interns, or clerks) could come in and eat what was left after the big-wigs had finished, but not the janitor that cleaned the meeting room.