Yes, that’s my criticism as well: a single adj category has no meaning because there is two of adjectives in Japanese, one with verbal trait, the other acting more like nouns. The difference can and needs to be addressed in a grammar.

The grandparent point about recursive rule(s) is valid however and their easiness is one of my favorite things in Japanese grammar.

Anyway, the problem seems easy at first but it’s totally possible to vaste hours perfecting a grammar that still doesn’t capture frequently occurring construction (been there done that).

The original article mentions something that I wish I had known when I began learning Japanese ~20 years ago: there aren’t really adjectives in Japanese, just nouns and verbs which have a few different inflection rules. Generally, any noun or verb (phrase) can modify another so there’s no need to create a separate category called adjective.

This is substantially true of English as well; we can construct a grammatically valid phrase such as "the sleepy gorillaesque lamppost".

With, of course, some exceptions. I'm confident there are exceptions in Japanese as well, being as exceptions are the rule in natural languages.

You can think of them as verb-like adjectives and noun-like adjectives, or you can think of them as adjective-like verbs and adjective-like nouns, but it really makes no difference one way or the other, and either way they are going to be their own separate categories.

... except for the fact that 形容詞 (i-adjectives) and 形容動詞 (na-adjectives) are used differently from 名詞 (nouns) and 動詞 (verbs). So there needs to be some distinction at some point, even if at the high level, 名詞 and 形容動詞 are similarish, and 形容詞 and 動詞 are similarish. But that doesn't go beyond -ish, and it's important to know.