Not a principle then.
Tohei had principals and whether you agree or disagree with them, they are principals. They are principals in the sense that one can validate or correct their (Aikido) activity vis a vis a principle : did I kept my weight underside - not so much, will work on that.
Not disagreeing with Tohei, and I meant no disrespect though I admit I was flippant.
Ultimately I feel some Iwama Kihon, and the basics in tegatana in Shodokan, and exercises in Yoshinkan like Hiriki no Yosei all kinda point in the same direction. The principle is mind and body coordinated, and alignment, and removal of extraneous muscle. So, eventually it would be part of "extend Ki" and the sensation of "weight underside" can lead concretely to shoulders lowering and biceps loosening.
It is very much a fundamental signpost to get people in the right direction and a piece of a principle but, reading Tohei's writing, it wasn't supposed to be a technique or a whole thing in and of itself. This was introducing mind and body coordination to a single isolated joint. It was meant to be something anyone could pick up, experiment with and realize in a couple of minutes.
Unbendable arm to technique is a little like turning a key is to driving IMO. No way to avoid it, absolutely essential no matter how you obtain it, but still not necessarily driving.