I'm a doctor so I can think of this usage. And trust me, we as students used to use a lot of such phrases just to have fun using medical jargon! This included, that is beyond your gray matter- beyond your thinking, paralyzed orbicularis oris -someone who never smiles! and a lot more. Ah, I just had nostalgia!
Okay, to begin with...
myalgia is pain in one or more muscle but here, in this context the pain is important and not the muscle because it's about the author's brain. The human brain is not required to 'move' and thus, our brain does not contain any muscle.
Now back to the sarcastic comment in which the author has played a pun. As you said that the intention is to 'I am skipping a lot of gritty detail' which means you don't want to bother your brain much about it though you know the truth.
The word spherical is used for the brain. We doctors use spheres to refer to the human brain.
So, to rephrase and reverse a 'pun' played on it, the sentence means though the things are right, the author simply wants to save his/er brain from pain (or bothering).