I'm not surprised that he has trouble understanding that. The 'meaning' of the poster frame isn't clear from that conversation. Does he want a description of what's being displayed in just the poster frame? How does this differ from the title of the video?

For example:

Title: Cat jumping on bed

Frame: Image of cat jumping towards bed

Video: Cat jumps toward bed and lands on it.

The vast majority of the time, the poster frame isn't going to be different than the title of the video.

Now, I agree that when it is different, there should be a way to specify an alt text for it. But to require it when it's the same as the title seems silly to me.