The notion that D&D is a combat game in which all else doesn't count is a trope, a wrong one, and one people here constantly work to correct.
A DM who wants to award XP based on killing blows has fallen into this trope and should be corrected.
But for your presence, the minion would not have cast the fatal fireball. The group is the author of the situation which kills the monster, so credit should be given.
I once threw a beholder at a level 1 party. The beholder was a questgiver and the encounter was on rails, but if they had found an extraordinary way of interacting with that monster, I would have awarded XP. Obviously they couldn't have killed it.