This is a good case of causation and correlation being conflated. Of course T-Mobile users will report problems with YouTube even if it’s just their carrier. Any localized ISP problem will show up in outage reports for top websites because that’s what everyone uses.
I mean, the thing is.. they can all get to downdetector.. so :)
That is true but I will also note that Facebook Messenger died for me across devices (even on the browser regular Facebook was fine but my messages would not load), and the email discussion I've seen largely says something similar- only Messenger failing for a lot of people around the same time a couple hours ago.