Possibly Roberts is sending a message that going forward with an impeachment trial is moot because President Trump is no longer the sitting president?
I think your correct, this is the legal argument that Professor Dershowitz has been making, Impeachment is to remove a sitting president for a crime committed while in office.
So it is good to see the Scalia philosophy of interpreting the Constitution as it is written is the first best approach when it clearly speaks on a subject.
Methinks this a "fig leaf" to cover his utter distaste and loathing for commoners whose only representation in recent times has been Trump.
This was done to make SCOTUS look impartial when they will refuse to intervene on Trump's behalf with the majority citing something about "Separation of Powers" and "Impeachment is a purely political matter", thus allowing the Senate to Convict and bar Trump from running for Federal office again.
My guess is that Alito and Thomas and possibly Barrent will be the dissenters when SCOTUS allows the trial to go forward.
The court doesn't send such messages. It ruled on the issue at hand.