Didn't they also block other services, like Google Maps, too? (IIRC there were webview wrapper apps that faked the user-agent.)
I think Maps was still usable through the browser unlike YouTube. And I don't think there was ever an official Maps app. At the time Microsoft's own maps were about as good...or rather Maps was often equally bad in equally surprising ways.
It wasn't working. At least on my device MS edge became super buggy and unusable whenever I went to maps.
Was youtube still flash based at the time? If so then that would make sense.
It had been serving it the WP7 app until Google started blocking it. At the same time Google blocked the WP7 browser on YouTube. There was nothing technical. Google was still serving up YouTube videos to iPhones.