Yes, USB extenders are not spec-legal (because the device isn't built expecting to be extended).
But you can have an extension cord which accepts USB on one end but doesn't accept USB on the other.
So the keyboard has a superset connector so that it can go in regular USB and notched USB, because it is verified to work right when using the extension cord.
This design also means you can't plug one extension cord into another to get an even longer distance (which the keyboard wouldn't expect). Pretty clever solution.
But you can have an extension cord which accepts USB on one end but doesn't accept USB on the other.
So the keyboard has a superset connector so that it can go in regular USB and notched USB, because it is verified to work right when using the extension cord.
This design also means you can't plug one extension cord into another to get an even longer distance (which the keyboard wouldn't expect). Pretty clever solution.