Isonomia is a really juicy topic, and yet another one that baffles me. It would make a great thread on its own; in fact I think we were discussing it not too long ago.
My take is that PS18-22 are discussing a finite human life in contrast to infinite time. Infinite time, space, quantities &c seem to me to be a different topic from this. Isonomia seems to be something implied by infinity, but I'm not even sure about that as I think the usage attributed to Epicurus is different from the common Greek usage.