my wife was playing the guild mages, played united defence on the objective card, spent 3 menace tokens for warp blast and declared the battle and competitive game won
I think that this is correct but am not 100% sure
Did you roll for the humans?
The Fome Mage's warp blast gives them a crushing victory, not an outright win. If the defence can Hail Mary it and roll a crushing victory themselves they can tie the fight round. Because if both sides have a crushing victory it is considered a tie.
I like using the united defence/we come in peace instant win button. But what's even better is when someone thinks they have you over a barrel with a united defence, and you have that we come in peace card already in your hand.