As far as the essential chairness of it goes, though, sitting in it describes it rather eloquently, imo.
Yeah, that's what a Zen master would say because Zen is concerned primarily with utility. The Zen master is the most biased one in the room, but at least he acknowledges the existence of other viewpoints. He validates their existence even though he his is superior.
Plato doesn’t get this far. He presumes a unitary truth to which all others are falsities. Not merely worse, but actual non-truths.
Take the chair. You’ve pretty accurately described the essential part most interesting to the Zen master and, probably, to the Platonic ideal of the chair that you can sit your fat patoot on it. But that doesn’t describe the totality of the chair.
What about the artistry of the chair? Does the simple utility truth apply to a chair that could be sat in, but also represents a greater artistic comment?
Sure, you can sit in the chair, but is that really what it is there for?
Then there’s a throne. Say, an empty throne that isn’t meant to be sat upon but is intended to be empty.
That’s obviously a chair, but its purpose is manifestly not to be sat upon. Under the Platonic idea, the throne is a non-chair.
Plato’s fault is that he views the truth as a unitary idea. There isn’t one truth to chairness, there are instead a number of facets to chairs. Instead of a single truth, there are a number of descriptions of the chair. Without one truth, the scope of wrongness is reduced and the possibility of knowing is increased. Where you recognize multiple facets you understand that there are many different possible ways to view a thing. That’s not to say at all that any one way is better than another (that’s the job of our Zen master), but the multiplicity of facets means that talking about one unitary truth isn’t useful.