They have not endorsed him (as of this morning at least). They all pledged to endorse the nominee, remember? The plan was to screw Trump out of potentially running 3rd party but, like so many of these central planning schemes, it has backfired on them.
Now Trump is saying “Fulfill your promise or else”, which he has every right to do as you can be sure they’d be saying to him if the situations were reversed.
TECHNICALLY they can wait till the vote goes down and still keep the promise. And fine. But TECHNICALLY then Donald could already pick other people.
Everybody’s mugging for the camera, it seems.