It does look like it has some sort of white armor tho tbh If we didn't know it was the sunne/sun pokemon a lot of us wouldn't have guessed fire type, and not only that, the sun isn't made of fire of any sort, it's a gas, obviously. So it kinda does fit a little. I guess fire is a natural chemical reaction to the gasses, so Solgaleo learning Fire moves would make even more sense.