I think Pierre was a little shocked that this pilot used TCAS as the sole source of traffic avoidance, surely we all know that their accuracy isn't perfect as a radar display so I think a compliance to the Controllers avoidance would have been safer.
Does Pierre know for sure that the aircraft was in cloud? In cloud, relying on TCAS separation alone seems unwise if improving horizontal separation is a possibility. But in visual consitions, it doesn't seem unreasonable that a crew would expect to be able to see and avoid an aircraft shown on TCAS, even if they hadn't sighted it at the time the avoidance advice was refused.