That's what I thought.
"Resemblance," yes. But since there is usually no one near at the moment he takes that posture, the ukes having just flown some distance away, why would he assume the posture of tenchinage?
Well tenshinage is a technique. I used to be taught different explanations as to what to do depending on how good I got at it. Basically from leading ki down and up, in two directions, form centre. Then onto taking into account centre line. Then how it's taking the mind up and down ie: splitting the mind etc. Eventually it became opening up. Opening up the opponent through being willing to open up yourself. This led to more reality on operating from hara.
From hara you feel the yin and yang, the taking in infinitely and the expansion out.
Now sitting comfortanly and letting the body move with it your arms and hands may go out sideways as if welcoming the universe. However I find when I am focused foreward yet returning to hara the the arms do as you describe. The feeling is very similar if not the same as tenshinage. Thus I could say tenshinage is returning to basic posture or say it in some profound way.
That's my take on it.
Now if you want to see me doing it, which I find myself doing to different degrees and only more pronounced depending on the circumstances you can watch a video of mine. Not saying anything to do with anything else just that I tend to do it or similar to it naturally at times. (video: spirit of loving protection 2, from 2:35 onwards)