Can't connect != Receives junk from the legitimate server. In that case, "junk" was an empty response.

They did test the game with an unavailable ad server, but it was still brittle (as in: not tested with badly formatted ads)

This is something they want to explicitly stop when they can show ads to a user base, so why make a fallback that allows users to pihole the ads?

You're assuming their motivations. As stated in the Twitter posts, the issue only occurred when a connection to the server was established, but the server returned an empty response. Microsoft fixed the issue after being contacted about it by making the servers completely refuse connections.