Hm. Context matters...
If that were at my table, I'd have ruled that he probably could use the power on the door ("You want to outline the door in purple light? Sure!") but would have warned the player there's no actual fire involved in the power so it would not have the desired effect, because really, the character would know that.
However, that wasn't happening at my campaign-table at home. That was part of a (one shot, with newbie players?) game with something like minor celebrities, being filmed for PR and marketing. That's a different situation.
My normal ruling might make the minor celebrity look a little dumb on camera (he hadn't bothered to read his power beyond the name?). Plus I risk making my game look a little nonsensical ("Why is it named Darkfire if there's no fire?"). So, the DM instead went with a ruling that is technically accurate (the choice to allow the power to be used on objects is in the DM's hands), and sits in a rules-detail where it isn't all that embarrassing if a new player doesn't know it.
In addition, in the filming context, when the DM knows there's a way to deal with it that's going to be bleeding obvious in 30 seconds, it makes sense to use a ruling that will keep the players from rooting around through their powers for several minutes looking for a solution, and instead point them at looking at the surroundings. While maybe not the best ruling in terms of getting players to do new things specifically with their powers, it does put emphasis that the game calls for interacting with the environment in ways the audience might not expect.
So, maybe in that context, it doesn't look all that bad...