I haven't seen the interview or read the book.
But the only question is "does the person (in this case, King Charles but it could be anybody) believe in Jesus Christ and that he was God?"
If yes - antichrist fail.
The anti Christ will, funnily enough, be
anti Christ - not for him. He will not be able to refer to, proclaim or worship Christ.
It was only a few years ago that someone say that Charles was the antichrist because he had a flag with a dragon on it.
Several leaders throughout history have been said to be the antichrist; all have been wrong.