Re: "stance of heaven (and earth)" and IS
To somewhat adjoin to the "definitions" and "sayings" discussion that's happening - while tying it back to the original thread title, here's some of how I look at "this stuff" (a lot I've been taught, some I've worked out, all is an ongoing exploration).
There's basically two natural forces at work on your body at any time: gravity pulling you down (Ki of Heaven) and the ground pushing you up (Ki of Earth). You can manipulate to a degree how your body relays these forces inside yourself - just via training your intent, which is something of a skill. You can further condition your body to better propagate these two directional forces (there's many different methods of doing this around a basically similar core set of principles) while also connecting your insides to build an unusual kind of strength. These things get combined over time and can give a significant functional power advantage in martial arts and sport - but by themselves do not necessarily replace functional application skill in martial arts and sports.
A side note - power here should be taken in a few ways. Power as in raw strength, is certainly there because you're using more of the whole body together as a connected unit, taking advantage of the natural elasticity of the ligaments and tendons, in addition to the raw strength of the bones and muscles - functioning as a connected and collective whole (again, caveats on purity of development notwithstanding). It's also a kind of "hidden" strength because it is less observable on the outside what's happening unless you know what to look for or feel- and even then can be masked in other ways (note: there's a reason so many traditional folks do demos wearing loose fitting kimono robey type things).
So there's a skill component and a conditioning component. Gross oversimplification, and there's lots of degrees of purity vs. application - depending on your approach - but this SHOULD (in my opinion) be thought of as very foundational work done to rewire the body and mind in order to better practice any martial art. And there may be advantages to one approach over another in terms of how it's packaged into a specific martial art or sport (performance enhancement vs. speed to market, etc.) -- but until even the basics are more widely practiced (if that is even feasible or likely), it's kind of a moot discussion on a broad level until there's a wider/deeper baseline level of skill/conditioning that more have achieved.
And all this aside - it doesn't mean just because you have IS skill/development you are a marvelous aikido practitioner, or MMA competitor, or BJJ player. There's absolutely advantages that are gained AND at least a number of traditional arts basically call out the conditioning/skills as the core component (AI-KI-DO anybody?). And if you're looking at the sayings of Ueshiba, I agree very strongly with what others have stated regarding comparing them back to Chinese sayings -- it does appear he was making his own case for "See, I get it, I'm in the club, too" with regard to tying his practice to the greater understanding of yin/yang, in/yo, etc. that was fundamental to the cosmology in Asian philosophy/culture - and externalized (pardon the pun) into a very practical form & application.
As always, FWIW and YMMV