"A separate version for the Apple Watch would remain [in the App Store], but then Apple pulled that one as well, telling Eleftheriou that keyboards aren’t allowed on the Apple Watch."
Im not familiar with all the history nor what they mean there, but this is a better source and i think explains the story better. I havent seen the dev make that claim at least.
https://arstechnica.com/tech-policy/2021/08/iphone-keyboard-...