No, it's not missing the point at all. The professor in this story is making an invalid assumption that all of their students have met the custodian. It is atypical for custodial staff to perform their duties during normal business hours. Custodians typically work during the evening. If you were a student who had a 10:00AM class in the building and no other classes in that particular building and the custodian starts cleaning at 5:00PM, it is very likely that you have never met them. The pop quiz question that faults you for poor social graces does so on the baseless, invalid assumption that you have met the custodian. As the old saying goes, when you ASSUME, you make an ASS out of U and ME.