It's been said multiple times, but if it was really that big of an issue, Fnatic should have paused the game. Yes, in the very middle of the teamfight. Why? Because it wouldn't have mattered and the officials probably would have done something like reset the situations (make everyone return to the fountain), or even reset the match if the former suggestion wasn't plausible.
They didn't call for a reset because they were still very confident they were going to win the game, and it came back and bit them in the ass. Even so, they made an incredibly poor decision to actually engage that teamfight after Sejuani was so low. She landed a multi-target ult and could have just disengaged while the rest of the team, which hadn't taken a single hit, could have kept SKT from pulling Baron.
Instead, they over-committed to a bad situation and, in the end, lost the game because of it. So yes, while that "bug" was unfortunate, it didn't cost them the game. Fnatic's decision to keep pushing the play, even after they got put on their heels from that unfortunate occurrence, lost them the game.
There's no need to make excuses for these guys, especially since they're playing SKT in the playoffs (and beating them the one time wasn't going to knock SKT out of the playoffs, in the first place). Professionals make mistakes, too. It happens. You want your remake? Here it is.