> the ONLY predicate that can be expressed about a single character is it being a member of a subset of all Unicode characters

You seem to be assuming that the comprehension would be purposefully restricted to iterating over strings. The original author already provided examples with predicates that don't involve checking for a subset of characters.

    old = [0, 1, None, 2]
    new = c"str(x + 1) for x in old if isinstance(x, int)"

The existing "".join() idiom isn't restricted to iterating over an existing string. You also have to account for nested comprehensions. There's nothing that would prevent you from having arbitrary complexity in string comprehension predicates, just like nothing prevents you from having arbitrary predicates when you join a generator expression.
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