Out of trim said "..good practice for QRA crews"....Yes, exactly! Perhaps getting closer to real reason?
He also said
Apart from losing comms with ATC, it would appear the crew concerned also deviated from the Flight Planned route
That sounds like ample justification for an intercept, and I would expect that the priority after that was the need to have a comprehensive debrief of the pilots concerned once on the ground, even if at the cost of a diversion.