It only assumes that they weren’t much more strongly selected for originally than they are now, lack of selection is just a special case of cuteness matching being hard. You wouldn’t expect there to have been a perfect match between cuteness sensing and cuteness causing features unless it had been selected for, so expecting the commom ancestor to be exceptionally cute implies sufficiently strong selection then, but not now or in between.
It only assumes that they weren’t much more strongly selected for originally than they are now, lack of selection is just a special case of cuteness matching being hard. You wouldn’t expect there to have been a perfect match between cuteness sensing and cuteness causing features unless it had been selected for, so expecting the commom ancestor to be exceptionally cute implies sufficiently strong selection then, but not now or in between.