I still have no idea what a "sleeve cap ease" is, but reading that post is fascinatingly esoteric.
"Ease" refers to the extra fabric in the sleeve cap. When I was taught to sew, I was taught that the sleeve cap should be larger that the arm hole, and then you have to do a whole bunch of cinching and stretching to get to sleeve sewn on correctly. The goal is for the sleeve cap to accommodate the shoulder without looking bunched or poofy, but (at least for novixe me) my sleeve seams always looked weird and messy.