I don't view Frum or Kristol as grifters. Frum is a conservative intellectual, and neoconservative who can be somewhat reasonable on issues that don't involve bombing Brown people in the Middle East. Kristol is a war monger neoconservative.
I think both view Trump as a long term liability. Trump has largely maintained the US military empire but hasn't started new conflicts, which of course concerns neoconservatives. At home he has no basic interest in conservative economic policies outside of tax cuts. He's an isolationist and idiot, two things that some on the "intellectual" right loath. So I get why they hate Trump.