if he says “correctly....on constitutional grounds” then he limits the playing field. If he wants to talk about justifying on any other grounds, then he needds to say so. IMHO, the question is not as clever as he apparently thinks it is. I suppose the Prez couldn’t veto his own impeachment, but it requires a veto-proof majority to vote for it anyway doesn’t it, even if it were a bill?
When he writes "correctly," I think he means "properly," or "in accordance with."
What you are implying, and outright asking in your first posting, is whether there are bills which the president cannot veto. I don't believe that to be the case, however a legal scholar may prove me wrong.
At any rate, that was not George Will's contention in the article.