I read somewhere else that O Sensei once said "Aikido is Irimi Atemi".
I can't say that I know the quote, or the authenticity of it. But I believe it could be an expression for the entering and strike that Osensei very often started his techniques with. The irimi step to a hanmi position, avoiding uke's attack, and at the same time getting close enough to strike.
If you learn this entrance well, there is really no need for anything more.
Nishio sensei stressed it: By the very first step, tori has already won - avoiding the attack and entering to a superior position. The atemi is a way of telling uke just that: Look, you have lost, you are at my mercy.
To Nishio sensei, the rest of the aikido technique was a way of sort of forgiving the attacker, and bringing everything to a peaceful conclusion.
Nishio sensei was very firm about making aikido something different from just being victorious in a battle. He constructed his techniques - both unarmed and armed versions - so that they contained multiple opportunities for uke to stop attacking and retreat.
I am also reminded about Tamura sensei's entrances. He is extremely distinct in his irimi entering, and makes sure that he has complete control of the situation at that very moment. So, for him as well as for Nishio sensei, the entrance step is decisive.
Maybe that was what Osensei meant?