This is interesting. A timeline I read says that the fire broke out at 6:50 pm which would mean that this Mass was still going on. I also read comments (maybe on twitter) by a man who said that he was at Mass when everyone there was asked to leave due to a fire. All the news reports I have read have said that all visitors had left the cathedral 5 (or 10) minutes before the fire broke out, making it seem like the Cathedral was empty of people. I don't know what to make of this. Either way, it is a very sad day for the people of Paris and the world.
The 6:30 pm Mass with the Archbishop is on Sundays. But according to the “Paris City Vision” site I got when I googled, there was supposed to be a 6:15 pm Mass today. So your point still stands.