In canon, the Iron Orcs are described as "reinforcing" the normal orc hordes, and are compared to Black Orcs, who, in gameplay rules, are an elite unit—you don't get an entire army of Black Orcs, the same way you don't get an entire army of Ironbreakers.
And again, I am not suggesting we purge the mountains of every greenskin in them, I am suggesting that we scout the area for actionable information—such as the location of their stronghold, the nature of their leadership etc—and then hand off the information to the Duke and maybe the local dwarf hold, who can then send an army to deal with them. This is exactly the same thing we did in Ostermark. Find out what's happening, find someone in authority, and make it their problem.
And please don't give me that "But Boney could have changed canon" excuse. The Iron Orcs are from a tier three canon source, and currently there are no tier two or one sources that contradict this.
And if the Iron Orcs are an entire faction, on the scale of a Greenskin tribe or a Skaven Clan, then frankly Bretonnia asking us to deal with them is a frank insult. "We'll only help you if you do this impossible task by yourself first" is a pretty rude way of saying "We'll never help you".
But either way, we'll never know this unless we spend that singular AP to scout and investigate—I am not demanding that we commit to a full extermination campaign, just to discover the basic facts and report them to someone who can orchestrate such a campaign.