Chief Justice Roberts is theoretically correct.
You are clearly correct.
Roberts almost certainly wasn't referring to the politics involved in the selection and approval of who sits on the bench, but rather the professional standard that a sitting judge should not bear any bias in favor of those who appointed him/her.
However, his subtle slam on Trump suggests he harbors some animosity resulting from Trump's earlier criticism. That is unfortunate for us and for Trump.
“Chief Justice Roberts is theoretically correct.”
But, in practice, Trump is right.