I think he used to play striker at youth level or something? Would explain. He scores goals and is a good nuisance when he goes forward (to be fair Tarkowski looks good in the attack too).
Apparently everyone in training buzzes about Keane as a great finisher, and while it used to be relatively normal for center backs to step in as strikers when needed due to injury, it's completely laughed at these days. I think Dyche started playing him last season in part for his ability to score, but would never consider playing him in a forward position, even in desperation.
On the other hand, it's a bit funny that when asked about it last season, Dyche scoffed at the notion (said something like, "playing a center half as a center forward? That's a new one."), but he also criticized the rigidity of continental football in talking about Onana's development, saying they tend to pigeonhole players and decide that you're going to be a certain kind of player and that's it, and so they're working on developing more aspects of Onana's game and eventually getting him into attack.