They don't, but that doesn't mean that they can't possibly improve.
The best pianist in the world still practices; the best tennis player in the world still has a coach. Now ask yourself "how does a tennis player get the level of grand slam finals if they don't have basic skills".
Show me the world's best at... anything - and I'll show you someone who isn't satisfied with their level and wants to get even better.
But, skills coach won't be about positions. Surely the clue is in the name, the skills coach should be about skills. Things like catching, passing, kicking, fending, hell, even running is a skill that can be improved.
The way it sounds to me is that the forwards, backs, defence coach at this level, mostly coaches the decision making, with a little on technique; the skills coach mostly coaches technique, with probably some input back into the overall strategy on decision making.