Earlier quoted context omitted.
That's why we need DNS over HTTPS and Encrypted Client Hello to become widely used. Then there would be no way of knowing, as long as Netflix uses the same infrastructure to serve fast.com and netflix.com
That wouldn't really help; the network activity signature of a speed test is noticably different from browsing or video streaming, and even if it wasn't, the connection for streaming lasts a lot longer than the test. You could defeat it with a timer.
Yes you could do a timer, but that would mean e.g. first 30s of all Netflix streams to be much faster and then slow down just in case they were in fact speed tests. Also would be difficult to differentiate between running a speed test vs a stream that gets its connection closed and reconnects.
All of this is an order of magnitude for ISPs to deal with than unthrottling traffic for specific domains. It doesn’t have to be perfect, but making it prohibitively more expensive helps a lot.