OK, so maybe technically under the hood, the SP doesnt really exist and it is in a constant amount of "vests" and if you never did any curating, posting or commenting, then the vests you have will stay the same. However, even in this extreme case of a non-community participant, your SP is still compounding since the SP is getting 90% of the new STEEM that is generated and that is based on overall marketcap.

Now if you want to talk about vests and how they dont compound for anybody that is totally inactive in the community, I could see how that case doesnt compound, but who cares about that special case for something that cant be exchanged?

If we contemplate the value of VESTS measured in STEEM units, then clearly we can see that when 9 new STEEM POWER are created for every 1 new STEEM (and given that STEEM POWER are 1:1 convertible to STEEM and the new STEEM POWER are distributed to existing SP holders), then clearly we can see that (the value that) VESTS (represents) have been transferred from STEEM holders to STEEM POWER holders. And that interest payment compounds the next time this done.