McConnell and Graham have said several times, in various ways, that they do not want material testimony at the trial. They may not even want any testimony of any kind, and they could make that happen, which is what many of us (you included, of course) have banged on about for some time: the actual democratic institutions within US (and UK) government are folding before our eyes.
Not even during the Clinton impeachment was there this much sycophancy (not even close, if you actually compare the process to what we are currently seeing).
Bolton’s statement specifically cites the court decision to throw out the Kupperman case and then specifically mentions “if the Senate”, so I am guessing he would say the House has forgone their right to subpoena (and it would have to be taken up again by the courts to force him to testify).