Reposting (& rewording) this since I think some people missed it:
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The big wide banner on the Smash site is split among multiple images, sliding repeatedly across like an image galery. I found the individual images, and images 02 and 03 do 'split' in-between Olimar & Little Mac. However, by searching for the images in the wayback machine, I just get a singular significant change corresponding to Sep. 13th, that is, the Isabelle reveal. If someone wants to further explore Wayback Machine, be my guest.
From what I can gather, the person in that leak loaded the site in a manner that the old image for Banner02 was loaded, and the newest image at the time for Banner03 was loaded, leading to the weird crop. The differences in the seam line up with the 4chan post when you connect those two images.
The 'old' images are there since like, September 10th as well, at least according to wayback, so it wasn't an immediate or reactive change.
Sep 13 - 21:09:
https://web.archive.org/web/20180913210950/https://www.smashbros.com/assets_v2/img/top/hero02_en.jpg
Sep 14 - 00:38:
https://web.archive.org/web/20180914003829/https://www.smashbros.com/assets_v2/img/top/hero02_en.jpg
Sep 13 - 21:09:
https://web.archive.org/web/20180913210951/https://www.smashbros.com/assets_v2/img/top/hero03_en.jpg
Sep 14 - 00:38:
https://web.archive.org/web/20180914003829/https://www.smashbros.com/assets_v2/img/top/hero03_en.jpg
This is what you get when you match up the first image for banner 02 with the second image for banner 03
Which seems to line up pretty well with the 4chan post.
It's a *massive* coincidence (and not in the sense that it 'proves' anything), particularly since I don't know how you could be extremely unlucky to load one old image and then one new one and then have it line up one month later with the Isaac thing that was apparently speculated (still ??? at that) to be in the same direct, but it's looking like nothing more than a coincidence.