No, API-wise the on-screen keyboard generates keystrokes for emoji (astral keystokes, multiple keystokes for multi-scalar-value emoji) even to an IME-aware app. In contrast, the emoji picker built into the Windows 10 Pinyin IME enters emoji via IME API.
> Under Settings > Devices > Typing > Hardware Keyboard you can turn on the IME "Show text suggestions as I type" even on a hardware keyboard in Windows 10, which gives you mobile-style auto-suggestions (you can also turn on mobile-style autocorrect even on a hardware keyboard).
Thanks!
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.
Yes, the signs that even English is passing through Windows' IME infrastructure in Windows 10 is pretty minimal by default (in Desktop mode, Tablet mode immediately turns on a couple more), but at this point Windows 10 makes almost all of it opt-in. Some of it is referred to as accessibility tools from an English perspective, because IMEs are also useful for accessibility.
The "big one" IME for most English users is the Emoji Keyboard accessible with Win+. or Win+; (whichever you prefer). It's really interesting how well emoji have helped Latin script users with further understanding the complexities of Unicode, fixing old ugly bugs in Unicode handling, and even introducing some such users to an IME that they want to use (sometimes every day).
Under Settings > Devices > Typing > Hardware Keyboard you can turn on the IME "Show text suggestions as I type" even on a hardware keyboard in Windows 10, which gives you mobile-style auto-suggestions (you can also turn on mobile-style autocorrect even on a hardware keyboard).