Re: Redshirt Chekov
Says 'novels' right in the post you quoted, so not cannon. Above fan theory, but it's definitely not set in stone.
But since the actor wasn't hired onto Star Trek yet when Space Seed aired, and yet Khan recognizes him, "in-universe" he had to be on the ship somewhere and just not promoted up to bridge duty...
Perhaps, if I am very lucky, the feeble efforts of my lifetime will someday be noticed and maybe, in some small way, they will be acknowledged as the greatest works of genius ever created by man. ~Jack Handey