I'm not sure it necessarily means that he is actually stealing the Moment at that very...err...moment. It just means they're being notified then that there has been a security breach. No one said the breach was still in progress, which would have nailed this down.Except the General and Androgar are looking at the No More Carving when the alarms sound indicating the Doctor is stealing the Moment.
The thing is, when they discuss the High Council's plans there, it's said that they've "already failed" seeming to imply that the events of The End Of Time have already transpired. But in End itself, it is said that the Doctor already has the Moment. Of course, it's possible that we've got some timey-wimey Time Lord logic in play, I suppose. For instance, if the Visionary has foreseen the Doctor steal it, they might consider that as good as him already having done it. Or the General might be saying that if anything the High Council were going to do were going to work, we wouldn't still be here, ergo because Gallifrey is still in the line of fire anything they do must fail and its as good as having already happened. But that's all a bit speculative.