The minimalist reading of it is that it signals he’ll be from the Flake/McCain/Corker school in the Senate—willing to speak out more forcefully against Trump than his colleagues. The maximalist reading is that it signals an openness to a 2020 primary challenge by someone, perhaps himself.
If the latter is the correct reading, and I kind of doubt it is, I’m with Ben on the home page and don’t think it makes much sense, absent some radically changed dynamic. Unless Trump quits or is rejected by the party by something like acclamation, a serious primary challenge would, if it failed, simply make it less likely Trump could win the general and would, if successful, probably be a nomination not worth having for reasons I set out here.