Sorry, my mistake
Not trying and not Able are wildly different. He (and kringle, who might have actually been the one leading the hunt) NEEDED Harry to take it over, which is the whole reason they threw the fight; "redirecting or stopping the hunt" would have buggered that up entirely and negated the whole point of the subterfuge.
Of course he was going to Hunt on Halloween if summoned to the Mortal World. By the sound of CD he does that every year. But being summoned doesnt force him to suddenly become a BAD Hunter, which this would be by his own definition.
The only possible reason they would need Harry to take over the hunt is if they were unable to go after the Outsiders on their own initiative.
Erlking was quite a good hunter in DB, he clearly knew something was going on because he knew Harry raised Sue, but he neither stopped the hunt nor left Chicago.
If he was simply not concerned by the darkhallow because it was only going to kill his entire hunting party but not him, that would make him a terrible Hunter. Again, this leads us to there was nothing he could do.
Or, alternatively, maybe he was doing something. Maybe he was chasing one of the other necromancers so Harry could stop Cowl.