I can firmly say from experience if you are late, or anyone of your party is late they can and WILL enforce the rule of cancelling your ressie. Especially if it is a popular place.
Our experience at Ohana last Dec was this:
My best friend lives in Orlando, so when we are at WDW she meets us at Ohana for dinner.
We arrived 10 mins early for our adr, only she was caught in traffic somewhere on Disney property. She called and said she was hurrying and on her way but may not make it.
When they called to seat us asked where the rest of our party was and we explained. The hostess then told us we wouldn't be seated until the whole party was there and if they weren't there in 10 mins we would lose our ressie.
My friend showed up 2 mins before the 10 would've been up....DH went to tell them we were all there and they HAD already canceled us!!!
We had to wait for another 15 mins while DH discussed with the manager that we had been told we had 10 mins and were all there with 2 mins to spare.
We were seated but we had to fight for it.
Moral of the story....don't be late, don't even think about it because they will give your table away. Too many people waiting to pay them and not be late.