I think there's a big difference between being good at some part of the game, and being good at the game as a whole.
And even then, sure, you can have a very clear best player and a very clear worst player, but what about everyone in between? I'm not sure if it's possible to have a journey of improvement in the game when you don't have a way to measure it.
A lot of games have multiple dimensions of skill, though - eg in cricket there is batting and bowling.
In games that aren't scored - like RPGs - there may not be a strict measure of skill, and it becomes more a matter of judgement. I still think these judgements can be made, though.
That was a self-imposed rule, and that is very akin to very much "skilled play" proponents often being concerned with verisimilitude and/or realism.
To me, "realism" relates to your other post about "playing (with) the GM". The role of "realism" in "skilled play" - at least how it seems to me - is to permit the players to make inferences (i) about likely risks/threats, and (ii) about feasible strategies/solutions. If there is a mis-match between the GM's view and the players' view of what's realistic, then (i) and (ii) will break down. (And I've experienced this, years ago now, in tournament play.)
Which means that "realism" is really a bit of a red herring - or, rather, it's a proxy for the predictability of certain patterns. Other sorts of predictability will do just as well - to me, this seems to be what is going on with some of the more gonzo monsters in Gygax's D&D (eg lurkers above, trappers, ear seekers, etc): they're not realistic at all, and not predictable considered in the abstract, but I assume that they made sense in Gygax's game as part of a pattern of escalation between GM and players. Removed from that context, they aren't "fair" (because pretty arbitrary) and don't support skilled play.
If, as per your post, "realism" is something that the
player is imposing on themself - eg they won't deploy a certain approach because it's not realistic or would be metagaming - then I think we've departed from skilled play as Gygax and Pulsipher described it. Some other, aesthetic, consideration is intruding.
Well player "skill" in An RPG is just rules mastery and extreme min maxing.
In classic D&D, at least as I've experienced, probably the most important dimension of skill involves choosing spell load-out, and then choosing how to deploy spells. There is a modest element of rules mastery in that, but mostly it's closer to the sort of tactical judgements/intuitions that a good wargamer has. It's about apprehending the "horizon" of risks and possibilities, and making sound judgements about how to navigate through that.