I'm inclined to write the comment more like "Usually the equal() check is redundant, but in setop plans it may not be, since prepunion.c assigns ressortgroupref equal to the column resno without regard to whether that matches the topmost level's sortgrouprefs and without regard to whether any implicit coercions are added in the setop tree. We might have to clean that up someday; but for now, just ignore any false matches."
+1. It explains the situation much more clearly and accurately.
To make it easier to review, I've updated the patch to be so.