A "strop" could be anything and any grit.
However, most people will use a "strop" as a finishing step for sharpening knives. Most will use a leather strop, others will use balsa, or films, or newspaper, or Unicorn horn mashed into a smooth surface....
Most strops are loaded with a compound of some kind, some are not however.
Compounds can vary wildly, but most are 2 micron to 0.25 micron. 8,000 grit waterstone is like 2 micron....so a strop is typically going to be finer than an 8k stone....yes.
But, like I said....."strop" in itself doesn't refer to anything specific enough to be called finer than an 8k stone.