Maybe you 100% believe Dark Knight has no possibility of being a tank. Which is where you would be incorrect. That's all there is to it.
Well yes, there's a possibility. But people seem to be throwing around the idea that anyone who could possibly be a tank WILL be a tank... and if that were the case we'd probably have tanking dragoons and summoners too.

Personally I think the "trading life for damage" gimmick that's so fundamental to most interpretations of the DRK archetype in Final Fantasy makes it being a tank less likely.