He likely means that this type of world first is a difficult claim to prove. On this server alone there are multiple groups that single heal and seven man this fight easily because they sell runs. I think it's great that you all did this, it's certainly fun to twist things up with new group compositions, but considering the low health of the boss on this encounter and the ease with which some other groups have been killing it, it might be a stretch to assume it's a world first until you confirm with a lot of other FCs on multiple servers. I don't in any way wish to diminish your accomplishment, I haven't attempted that group composition yet. I just think it's a difficult claim to make without overwhelming evidence.