It become known that the group are Millwall because during general chit chat when buying the tickets, one of the group said something to the effect of "lucky we can get tickets here after our game was called off, our day isn't wasted". You were at home. Had you been away and MUFC were at home, then no doubt the group would have gone there. They just wanted to watch a game of football and were genuinely pleased to have an opportunity to visit the Etihad. I take your point about the history between your two clubs but equally, while I accept it's very unlikely, do you accept that there will be some City fans who could go to Old Trafford without causing a problem there? If the starting point is fans from rival clubs can't attend another ground without being a problem, isn't that a little depressing and just reinforces the notion that some have, which is all fans have the potential to be a problem, when that isn't the case at all?
A lot of fans wouldn't dream of going to another club when they have time on their hands but some just love football and would go anywhere to watch it. And a personal anecdote.. we support West Ham and our two young nephews are Chelsea fans. We've taken them to Stamford Bridge a couple of times and behaved, because we know how too, as do more people than don't.