I belive this functions like a VPN in some ways and blocks video throttling. They use traffic inspection to throttle video streams down to 480p unless you have the most premium of plans.
I've never heard that IPS (not the content provider) is throttling down Video Quality by altering the traffic. Do you have some links to back up that claim? This doesn't make much sense, as they would have to download the high quality video anyway, then invest massive CPU power to downscale this. Most content providers will scale down the quality if they detect bad network conditions. If ISP would want lower quality, they could just artifficaly slow the connection.
This has been happening for years. They just limit bandwidth to video sites, so your player selects the corresponding stream quality.
With Apple Relay this might get way harder, if not impossible.