The fast.com page kicks off requests to nflxvideo.net domains for the actual speed measurement. And it wouldn't surprise me if actual Netflix video streaming made occasional connections to fast.com purely to make it harder for ISPs to cheat.

You can think of the requests to fast.com as just loading the speed test control scripts and user interface. The actual speed test loads files from the same servers (with the same SNI hostname) used by actual Netflix video streaming. It wouldn't surprise me at all if the fast.com speed test loads real streaming video segments from these servers, the only difference being that it doesn't have the decryption key for these videos.

That's surely how I would have done it.

It would also not surprise me if dns requests for fast.com temporarily elevated bandwidth limits for netflix

It does. You're loading segments of real movies on Netflix that are not encumbered by copyright.

But they're intentionally otherwise indistinguishable from any other real movie on Netflix.

Yes a few ISPs actually do this (not just for Netflix, but for other sites in general).