The thing they have to fight though is that when you register for a trademark, it doesn't apply to every sector.

Absolutely true! Which is why it's good that they're both in the entertainment sector.
And I'm sure it's a non zero percentage of people who do think the fictional band is called Demon Hunters. But that may not even matter. There is a lot of overlap, being both in entertainment, and about music and bands etc. Hell, both are even about actual demons! This is a Christian metal band after all.