Stopped Home Sharing my iTunes library. Still shows up in Apple TV?

This is hardly a scientific opinion, but I would guess that what you are watching on the AppleTV post-Home Sharing shutdown has already either been queued up to watch (as in was already in progress) or was recently watched, so it is possibly stored in the section of the RAM reserved as a buffer for the streaming.

Also possible here is some kind of glitch where it might say in iTunes that Home Sharing is off when it is actually still on. I assume this is possible as sometimes my AppleTV will say Home Sharing is off when iTunes says it is on.

I dont know if this helps, but there's another section too.

under iTunes settings [this is with regards to the computer where the library resides]
iTunes>Preferences>Sharing

Is the "share my library on my local network" ticked?