Only Flashpoint is to blame imo. Sure, Dev1ce got mad on Twitter, but so what? He should have, this is a big enough tournament that shit like this matters.
If what NiP said is true and there were security settings enabled on the server that caused the packet loss, then it is entirely Flashpoint's fault and they should reschedule. This isn't the same as one player having bad internet in their home, or the org having a bad connection to their facility. That's on the player/team to figure out, or if its an ISP issue it's just bad luck. Given that they mentioned they had no issues connecting to other servers, it should have been obvious that it was a FaceIt/Flashpoint problem.
Also, Anonymo straight up lied on Twitter, saying that the decision was unsubstantiated and that the internet issues were not clear. Richard Lewis, someone who was chewing out Dev1ce for whining on Twitter only hours before, verified with FaceIt that the internet issues did indeed happen.
TOs usually have some rule in their rulebook that allows them to make decisions about cases that aren't explicitly accounted for. This is one of them.