How to determine if the presenter will attend as a session chair
I think you are misunderstanding the instructions.

Let's say a presenter is scheduled to begin their talk at 11:00.  If they are not there at 11:00, they have failed to attend, for the purposes of these instructions.  The instructions go on to say what you should do in that case.

It's not asking you to predict the future, or to guess at 11:00 whether they will show up at 11:15.  What happens at 11:15 is irrelevant.  If they weren't there at 11:00 they missed their talk.  Even if they show up later, it's too late.