Apparently, he is saying he doesn’t have the authority as per Constitution as he interprets it
But the Senate could come back and designate him giving him the authority
Then it would be back to him to refuse.
I would presume the Senate would normally not do that if they knew Roberts would refuse..but these are not normal times.
The Senate doesn't have that authority. The requirement that the Chief Justice presides when the President is tried is in Article II of the Constitution, which is Supreme Law of the Land. The Senate cannot override the text of the Constitution.
That said, the Senate could certainly request his presence, but it would be an admission that the Senate feels it cannot be fair and impartial on the matter. The Chief Justice cannot decline when the President is tried, but the Constitution doesn't say the Chief Justice cannot preside over lesser impeachments at the invitation of the Senate. If invited, he could still decline.
-PJ