Dude, he's quoting a classical Chinese text and saying that it is the secret of his Aikido. It doesn't get much more influential than that.
This is where I get off.
He didn't say it was the secret of Aikido. There is no secret. He pointed out to a person concerned what that particular person or group of persons were not understanding. He no doubt pointed out other principles to others when they were lacking them.
I can just see them now running away and expounding how he has told them the secret too.
Yin and yang is basically a no brainer rather than a secret. Unless of course you're not used to such spiritual concepts in which case it will be like some special secret.
Non resistance is a 'secret' of Aikido too, as is kokyu and universal love and the spirit of loving protection and of course Ki. Not forgetting the void...zero....koshi....of course. Many 'secrets''