I didn't mean that the information `cond-expand` provides is superfluous or can be ignored. I mean that it is not important that `cond-expand` is part of the Scheme language itself. It is enough if the installer/packager does understand it (or some similar mechanism).
I think I understand now. But if I do, this is only true if you are willing to insist that semi-portable code has to be passed through a packager before it can be used. With cond-expand as part of Scheme, I can say (import (foo bar)) in any of the supported Schemes, and it Just Works. Otherwise, to run my code on four Schemes, I'd need to run the packagers for all four of them to make the code work at all. With cond-expand as a library declaration and (when useful) an expression type, I don't.
Having a packager also understand cond-expand is useful for Schemes that do not support cond-expand or the foo.<impl>.sls approach.