Google Play is accessible only if you hook your Android device to a Google account AND you Andorid device vendor has put up the $$ to use the proprietary Google apps like Play.

The real question is why so many apps are distributed only through Play, excluding privacy-conscious users and open source Android devices.

Nobody said they shouldn't have provided an APK.

This is a common practice for such pre-releases. Though they could have added the APK on the Google Play store but then they will have to introduce a new internal API where they keep a check on the pre-release users. Can be done but I think this manual distribution can be used for many other things!

But it's annoying that they do not want to support rooted devices!

I'm also curious why they don't "support" rooted devices to the extent they try to detect it and refuse to start. It's not that there's anything needs to be done to support them - they're no different from any other devices, just provides end-user a method to gain uid 0.

Is Nokia hiding something or what?

I'm on my first Android phone, never installed any mods, and it seemed quite simple to me. Possibly even more simple than using Play.

The flow can be more simple, as you suggest, but what you lose is the assurance from Google that this app is from the publisher it's marked as being from and all of the surrounding mechanisms like push updates that it makes no sense to reimplement on a per app basis.