They seem to go a different route with a lot of the gym leader companions—they don't have to stick unilaterally to their type. For example, Brock had Vulpix and Crobat and Cilan had Stunfisk and Crustle.
Also your post confuses me a bit... Brawly is a Fighting gym leader, so that isn't exactly misleading.
I'd think that he means...
> The hiker tells you that you get flash to illuminate dark caves from this gym.
> This gym is dark as satan's armpit.
> Brawly goes on about training in 'this pitch-black cave'
> People're getting massive nerd boners
> It's a dark-type gym at last!
Then...
Fighting.
E: Oh, you thought it was... Water.... Never mind.