Well, Endo sensei does not use the term. He just talks of the palm and explains how relax the wrist and the fingers to make strong connection.
It is when you do something like nei gong (in my case) or talk ot someone who skilled in the chinese arts ... you come to realize, that Endo talks about using the laogong point.
If you look for "laogong" or Pericardium 8 (P8) you will easily find the point and descriptions where it is and what it is. (I myself locate it at the center of my palm.)
And yes, it is one of the most important points.
But again: Endo sensei does not speak about the "inner aspects" of this point. At least not during open seminars. He just teaches it to be technically usefull and how use the body to get the most out of it.
Knowing, that Endo understands aikidō as dao, listening carefully to his words and also knowing also, that he has some experience in doing tai chi I am sure he shows using laogong without refering to it.
Interesting enough that a lot of his students end up doing some daostic practice.
No, this is a different.
In this video
you can get at glimpse of the use of the thumb "when uke is holding strongly". And the connection through ukes laogong point "if your partner is holding lightly". (This is not comprehensiv, just to get a little impression.)
The use of the thumb, or to be more precise of the thumb basal joint, is - as far as I see - not "connected to daositic practice" or using an energy point but just using a technicall posibility of the human body.
As you see both ways to connect to uke and to move him are still important in Endo's aikidō.