Then how can he call this impeachment unconstitutional?
It’s unconstitutional without the Chief Justice officiating.
If Roberts agrees to preside, then it’s safe to assume that Trump is indeed the “President” (there must be some reason why the lawyers used quotation marks, I’m not sure why).
But in any event, this is just Kabuki Theater. There will not be 67 votes against him, and this will all go away.