That's fine. Just not gg, because it certainly was not.
but keep in mind if someone on the losing team rage-quitted halfway through the match it was probably because they were losing, meaning you'd probably have lost anyway (didn't you argue it was very hard to turn games around), so gg.
It's possible. I have a screenshot of a victory with an 0-9 rage quit AI feeder Rook on my side with the other team completely outleveled, outdamage, outbuilding/flag/gold/favor/everything'd us, and we still beat them.
Also, I've had people on the WINNING side get spontaneously disconnected or server split. You can easily turn it around by feeding on the AI.
So "situation went from bad to worse" isn't the only thing that happens.