Agree with others that grouping verbs into -ru and -u verbs is a weird choice that only kicks the can down the road. I get why a textbook like Genki does this, i.e., not wanting to throw terms like 「一段」and 「五段」at people literally a few chapters into their Japanese language learning journey, but it just muddies the waters.

Like so much pedagogy, this feels like a good example of "hey, here's a quick-and-dirty heuristic for how this thing works, but just keep in mind that we'll explain how it really works later on". Except Genki I+II never mentions the terms 「一段」or「五段」at all. (it's similarly criminal that they never introduce pitch accent either, but that's another story).

My advice to anyone getting started with Japanese would be to get out of the introductory textbook stage as quickly as possible. Supplement with grammar explanations from other resources early on. Cure Dolly, Imabi.net, Tofugu, YouTubers, LLMs--whatever clicks with you.

I'm just glad that early in my learning journey, I heard a YouTuber describe Japanese as "agglutinative" (with a comparison to German)--a term I'd never encountered before. A light bulb went off for me and things started to click.

In any event, glad to see this kind of article kick off discussion/debate :D. 皆さんは、頑張りましょう！