In art, are there some people whose skill level can never be reached?

Technically, no, because if someone reached that level, this means that level is potentially reachable.

In practice, some artists possess unique gifts that make them stand out above the rest of humans. For example, Stephen Wiltshire, who has the savant syndrome and can recreate complex landscapes from memory after seeing them just once:

That's not exactly skill; it's a gift. Skill can be trained and achieved; something like this, one has to be born with.

But anything one can learn can be reached by more than one and more than a handful.

The issue [I see] with your question is that you think art is about striving to reach unparalleled heights of skill. It's not a competition. Art is about finding the style, technique, subject, and approach that touch people's souls.

Veneration of pure skill is at the heart of what I consider the rot of art today, the obsession with “hyperrealistic portraits”, where people complete who can imitate a photocopy of someone else's photo the best. That's not art, similar to how Olympic sports are not healthy lifestyle.