Wait, WHERE were these placeholders added? I don't see any extras on the Smash Bros website.
Ok, so this made me go check the code myself. First, this is only in the code for the page where you can buy the DLC. It lines up with the screenshots provided in the resetera thread, but here's some facts:
Before, every character was contained in a <div> tag that was given an ID of the character's name. Now, they have enclosed each character in an additional <div> tag given a class "challengerBlade". This is extremely unlikely to be a codename, and there are not extra slots added to the website unless I'm overlooking them. There are four challengerBlade class divs that contain the three available fighters, and one for Piranha Plant.
However, this is probably an upgrade to the code to make it easier to add and move DLC characters as they are added. They might be adding some kind of feature to hide certain "blades" of characters, like putting them into accordion boxes or something.
tl;dr this is actually a false alarm, there are no additional "slots" on the website just yet.