They can.
There was a game in Germany where both teams had gone to their dressing rooms for half time and VAR decided there should be a penalty and they were brought out again for the penalty to be taken before going back in for half time.
Surely there's a difference between interrupting the match for HT break and the final whistle, finally ending the match.
I think the point is that a penalty can only be awarded for an offence that took place during the match, but time can (must) be added at half time or full time for it to be taken.
If the ref blows the final whistle, and then Fern gives a Leeds player a smack in the mouth, a penalty cannot be awarded.