Joining as a formation is one thing, but once in the visual circuit and downwind you are your own aircraft, flying your own pattern as required to fit in with all the other visual traffic and making your own calls. Radar / TCAS / Link / SKE / FLIR etc are all additional tools but it remains a visual circuit.
I agree with your concern with the non-standard R/T and that the Occurrence Investigation seemed to skirt around it. The Airprox Board said it more clearly:
The Board quickly agreed that the Lossiemouth Occurrence Investigation had been thorough and that relevant causal factors had been identified that did not warrant further detailed debate by the Board. That being said, although the Board agreed that there had been a series of cumulative factors that had led to this incident, they were struck by the fact that, having lost sight of and formed a flawed mental model about the position of his leader, it had been the reinforcement of that model by the ambiguous R/T phraseology that had been a key element in the chain of events.
In the Board’s opinion this ambiguous terminology was fundamentally flawed and had unsurprisingly reinforced Typhoon(2) pilot’s incorrect mental model...
No kidding.