The emoji keyboard may not be entirely using the IME API, but it does do some IME-like things even in English. The big thing I'm thinking of is the way it works when you type English words to search the emoji. I think it still most often defaults to passing the keys along to the application as well and replaces it with keyboard keystrokes or selection APIs, but I have seen it sometimes do the IME thing that the text you are typing is shown underlined and not sent to the application. Though in mentioning it, I don't recall the exact combination of app and emoji I was trying to find where I saw that happen or know precisely enough why it would vary in order to reproduce it just now, and maybe that was just a difference between early Insider versions of the emoji keyboard and current operations or something similar that I'm misremembering.

That said, even if it isn't using the IME APIs directly in most cases, it's still useful as a teaching tool/analogy tool/example tool to English speakers of what an IME can be like to use, even if a nice-to-have for an English writer versus a necessary required tool for other languages.