Are questions about Chromecast and its use on-topic, like this question, or do they belong on Android Enthusiasts?
Chromecast falls into the 'electronics gadget' category, which would be off-topic for Super User:
electronic devices, media players, cell phones or smart phones, except insofar as they interface with your computer,
To answer the second half of your question, there's no reason I can think of that the vast majority of Chromecast questions would belong on Android Enthusiasts.
The Chromecast devices don't run an Android OS (I think it's just the Chrome browser sitting on top of a generic Linux kernel, like the ChromeOS laptops) and they're operable from pretty much any modern OS (including iOS, Android, Windows, MacOS, and anything that can run Chrome) so I don't think there's anything Android specific with the Chromecast device.
If someone is hitting a specifically Android related problem streaming from an Android device to a Chromecast then that could be on-topic for AE, and obviously if and when there's an Android app released to control it, then questions about that app could be on-topic for AE. But you could say substitute "Android" for "iOS" or "Ubuntu" and so on all through that last sentence.