I'm really curious that people are having this reaction. I looked at the side-by-side pics and saw two fairly generic e-commerce layouts with a few small similarities, but just as many differences. My immediate reaction was, "Huh? There's no case here."
A lot of HN commenters are having the opposite reaction, not just you, and so I suppose you all might be onto something. But I still don't see it myself.
I have the same reaction you (and Garry Tan) do. Having a members-only upscale e-commerce wasn't invented by Fab, Gilt Groupe was doing it four years before Fab was founded. There aren't that many variations on what a members-only-bouncer page can look like.
Aside from the choice of layout of the product page, which itself is generic, I don't see anything in common. Having the time remaining in the header bar has been commonplace on flash sale sites (Groupon and clones) since before Fab was ever founded.
E-commerce layouts have been studied in depth, it wouldn't surprise me if the layout choices (like repeating the add to cart button at the top and bottom) by Fab were not arbitrary but based on known best-practices.
A lot of HN commenters are having the opposite reaction, not just you, and so I suppose you all might be onto something. But I still don't see it myself.