It just may be that control operator knew the distance in time from the certain coordinates on the radar where the bomb had to be released...
I do not know. Could be in those times that airplane just had to reach certain point, release the bomb and rush back. Everything else was monitored by radar and coordinated from some control center. For sure it could not be so precise like now.
And, of course, I could be talking nonsense, because I am not competent - this is just loud thinking, so do not judge me hard.