I assume you have got the wrong idea of backdoor. New map is default by backdoor protected map yet there are some heros which can outpush the limit but in the current case everything was normal. You must know when your opponent creep waves are inside your base, and every other lane towers are destroyed the other side sill can be raxed without being backdoor protected not only that they can also directly end the game if even 1 opponent creep is alive inside your base.
Elaborating the situation :
They already raxed your offlane. There was a creep wave inside your base while visage was pushing.So it can't be counted as backdoor.
Ps: Backdoor is allowed though unless there's some game breaking bugs.