How can a pronoun refer to a negative indefinite pronoun?

Question

Optional Foreword: I understand pronouns such as 'none, no one, nobody', if they're the subject of one independent clause. But the quote below (encountered herein) confuses me.

I ask here (and not on the English SE sites), because my ordeal with this problem also in French and Spanish, betrays a deeper linguistic problem, independent of a specific language.

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(TL;DR) [Original Source]
'category' should be used by no-one who is not prepared to state [the following]  [...].

I can't pinpoint the source of my confusion; so please advise. However, I suspect that I'm troubled by the use of who (the relative pronoun) to refer to no-one (its antecedent).
How can a pronoun refer to a nonexistent someone or something? Is this nonsensical?
In the sentence above, if 'category' should be used by no-one, then no one exists.
So how can you continue to consider this no-one, especially with a relative clause?

I already deduced, and so ask NOT about, the meaning of the above:
One should use 'category' only if prepared to state [the following].

Answer 1

Pronouns often don't have reference -- they have cross-reference. It is like the difference in high-school algebra between constants and variables. Constants are comparable to simple names of individuals, and when used, they have reference to those individuals.

Variables, like the "him" in
1. "Everyone wants someone to love him",
do not refer to individuals until a constant is substituted. Then that constant will be the reference:
2. "Horace wants someone to love him."

Pronouns can sometimes be used referentially, also. In 2, "him" refers to Horace.
1 is ambiguous in writing, since if the "him" is stressed, "him" could be used to refer to a certain person who is a desired object of affection in everyone's mind.

It is easier to discuss the referentiality of pronouns if we can use the notation of predicate logic and avoid distracting multiple interpretations.

Let's take an example similar to the one asked about: "No one who respects himself should eat his hat." The "one", "who", "himself", and "his" are cross-referenced, though of course no individual is referred to -- the sentence is a generality. Using a variable to get at the logic of this, we could write "No x such that x respects x should eat x's hat".

If we use restricted quantification, one more step gives us a logical form: "(Any x: x respects x) not(x should eat x's hat)".

In the linguistic literature, instead of the term cross-reference that I used above, one generally finds "coindex", or even "coreference", for the relationship between arguments represented by the same variable in a logical form. This is not the best terminology, since it seems to imply reference.