Apple gets your voice recordings which for various privacy laws they can’t resell. “Yay wiretap laws from the 50s”
As for typing, a quick search, apple support claims that the keyboard cache is only used for autocorrect purposes. How often you use a word and how you like to spell it.
Having it function as a keylogger would give them a headache, from data use, battery life, hackers exploiting it, etc.
And of course the warnings seem targeted at third party keyboards who can add on extra feature beyond IOS barebones.