Not sure what the big issue is here. People come in all sizes and have different positions, stride and arm reach on every throw. Most stance rules essentially impact players differently based on their size and reach. The RC didn't feel the need to specify an exact length where you get relief behind an object. Just whether you can take a stance. Argue with the RC if you feel the common sense application of this relief rule needs more precision because so many people are confused or taking advantage of it. To my knowledge it's never come up as an issue.
You asked if the 30cm had anything to do with relief when up against a solid object. I said 'no' it had to do with the max size of the disc. No contradiction in what I posted.
Rater of the tossed arc.