Are focusing adverbs exceptions of adverb definitions?

“Adverbs characteristically modify verbs and other categories except nouns, especially adjectives and adverbs.” (The Cambridge Grammar of the English Language, p.563)

“The basic division, then, is between words that modify nouns, and words that modify other categories (categories of words or of larger constituents). The noun modifiers are adjectives, and the others are adverbs.” (A Student’s Introduction to English Grammar by Huddleston & Pullum, p.123)

Even though adverbs are excluded, by said definition, from modifying nouns, the so-called focusing adverbs are found in CGEL and COCA, that modify nouns, proper nouns and pronouns.

You are the only boy.
Only Kim resigned.
Only he knows.

Can they be regarded as exceptions of the definition? Or is there a certain reason, for example, the focusing range being movable or sometimes ambiguous we cannot say definitely that the adverbs modify only nouns?

The key is in this passage:

[Adjectives are] words that modify nouns, and [adverbs are] words that modify other categories (categories of words or of larger constituents).

Here, only is modifying a "larger constituent"—namely, a noun phrase (NP). You have given examples where the noun phrase consists of one word, but if you replace the noun phrase with a multi-word one, you would see that only would have to apply to the entire phrase, and not just with a single noun.

Consider if we replaced the noun with a longer noun phrase:

Only [Kim] resigned → Only [Sarah and Kim] resigned.

Note that only modifies the whole phrase, and not just the noun itself.