You're not the only one to be taken aback by that. Yes, the Hugos do fantasy, but I'm not sure I would have even thought to categorize Sinners as fantasy.

I mean, yes, it has vampires. Was From Dusk till Dawn fantasy?

But as a sister comment suggests, categorization is never easy, and probably just reinforced personal biases. (See Old Town Road being reminded from Billboard's country music list.)