Jumping in late in the game but I was listening also. It was an ERU training session, easiest was to tell is because it was happening on the ERU talkgroup and not Div 1. If this was a real occurrence all units involved would have stayed on Div 1 and anybody not included in the call would have been switched to Special Events 2.
I have two theorieson why they do this:
1) The Uniformed Officers don't have access to the ERU channels
2) It is too risky to have the units involved in the high risk tactical call switch channels for fear of a unit getting lost in the switch or missing a life saving transmission during the switch.
My guess is 2.......