Keep in mind that there haven't been all that many iMacs taken apart to confirm that this is a continuing issue of not providing the SSD connector in the base model iMac. The iFixIt teardown was on one of the very first ones available for purchase, although you may have made your purchase around the same time. Was your's made in the USA?
Unfortunately, you really don't know until you slice it apart and see.
As has been pointed out, you do have through tomorrow 1/7 to return your iMac and purchase a Fusion one, now that Fusion is available in the base model.
Good luck ...
-howard