You are correct. The following line is new information added since I last visited that site:

"Confederate soldiers, who died or were killed earlier than Jan. I, 1863, or later than May I, 1863, are not included in this list, and no complete list of their names has been preserved."

That would mean that any casualties of Chickamauga would be marked as unknowns (or possibly not marked, at all).

This would account for some authorities believing that 2,500 Confederates are buried there, but only 887 are marked (other than about 26 postwar burials, of which only some are marked).