That's all optics, not HA. Q isn't even an acknowledged employee(s) of the government for that matter.
Ok, repeat after me "Mueller works for President Trump."
Once you really understand what that means, everything else makes sense (he's not going to subpoena the guy he works for).
Again, he works for Trump. He takes his direction from Trump. Think.
They simply provide a statement on the record. No subpoena is necessary. Talk of a subpoena is purely a distraction to keep the left chasing shiny things.
My guess, btw, is that Trump is listed as a CI in the CP FISA app. There's at least a track record of him working with the FBI since the 80s taking down mobsters in Atlantic City.. and then there was that "wealthy Atlantic City casino owner" mentioned somewhere…
If they use their position, sure. Something like "As a CIA agent I support…" would be bad. But "as Bob Jones, I support…" not so much.