That may be one factor, but it certainly isn't the only one. There are plenty of remote desktop apps on the App Store. But Steam Link was held up in review because Apple did not want people being able to buy games without getting a cut.
Microsoft "allows" people to install emulators in the same sense that Apple "allows" it on macOS, but a quick search confirms that Microsoft does not allow emulators on their app store, nor does Google. (These policies may specifically be for video game emulation, which is mostly what the legal grey area covers.)
Google play has emulators, here are some examples [1] [2]
It even has Bochs [3]
Or am I misunderstanding something?
[1] https://play.google.com/store/apps/details?id=com.explusalph...
[2] https://play.google.com/store/apps/details?id=org.mupen64plu...
[3] https://play.google.com/store/apps/details?id=net.sourceforg...
The issue is that Apple wants a big profit cut on each app. If you were allowed to download & run applications on your own computer, then Apple wouldn't get its cut.