Is Attack Of The Cybermen somewhat misnamed? There's not really an actual attack by the Cybermen in the story. They are in the sewers, and I guess you can say they attacked Lytton and his men there. But most of the Cybermen action in the story is about redirecting Halley's Comet into the Earth, which never happens, and the gathering of forces on Telos. You might say they attacked the indigenous people of Telos.
It's more like Impending Attack Of The Cybermen or Proposed Attack Of The Cybermen.
Their mission to redirect Haley's Comet into the Earth would have been their attack if it had been successful. The fact that this was prevented from happening is what formed a large part of the story.
However, they were still engaged on this mission, therefore their attack was still underway. It was just never successfully completed.
It's sort of like Earthshock. That title absolutely gives nothing away about the surprise return of the Cybermen at the end of part 1. Attack Of The Cybermen, though, kind of gives it away. Like in so many Of The Daleks stories. It gives away what villain is going to be appearing. Back in the black and white days when the episodes had individual titles, you could get away with surprise villain reveals like the Daleks at the end of the first episode of The Dalek Invasion Of Earth. The overall title given to it later gives it away, but the first episode title doesn't. However, it was probably all blown by the Radio Times advertising the return of the Daleks to make sure people would be watching.