Well we don’t know what happened before. But the article makes it pretty clear he grabbed the girl after they confronted him, threatened to stab her unless they let him go, they didn’t, so he did. So no they most definitely did NOT stop the girl from being stabbed.
Guy had already stabbed one of their friends for no reason. He was obviously off on a murderous, mental rampage and needed to be stopped by anyone who had the balls to take them on. If he wasn’t stopped there and then, he would probably have gone off and stabbed someone else, probably fatally.
Hard to say exactly what happened, but they do say her family was grateful, so it sounds like they perceived that these guys saved her.