Frum does embrace elements of Trumpism. He espouses greater restrictions on immigration, for example. And he would support a larger role for government in economic affairs than most conservatives - or at least those holding national public office like Paul Ryan - would be willing to accept.

Part of Frum's rejection of Trump is his larger break from the GOP during the Obama Administration. He argues that the GOP should give up repealing Obamacare & largely accept it as is. He also argues, along with Reformocons, the GOP's economic agenda is a relic of 1980s that should be abandoned (I think him right on both accounts).

To my knowledge, he remains a neoncon in foreign policy, however.