Pretty straightforward admonition. The key is to devote whatever time and mental resources it takes to really understand this via jujutsu and shiatsu, with lots of time practicing both as discreet parts of the system and exploring the nexus between the two -- solo and paired.
Glad you quoted it, as I would have done the same. About the only place "muscle" belongs in beginning the study of Hakkoryu is in one's wrist, and eventually we're supposed learn to negate that, too (I know, that sounds obscure). It took me a long time to embrace this, but it's key.
On a side note, we recently accepted a student who is built like like a brick outhouse (his trade is actually heavy masonry); you'd think he'd be the antithesis of relaxation. Turns out he has a significant background in a Chinese art called Wing Chun, and his ability to quickly learn correct posture and his ability to relax and "unuse" muscle is exemplary - he's going to be a quick study, I think. His biggest battle is proper use of his hands (grip) of all things, it's frustrating him a lot believe it or not!
The work you're doing to examine your jujutsu is exactly what every student should be doing to find out more. I don't know if you'll find what you're looking for with your school or not, as I don't have experience with them. Maybe compare notes with Richard Stevens who is also a member of your school with similar questions, he's also a member here.