I had a very similar experience there. A mysterious 4 letter acronym that made no sense even if you knew what it stood for. "Multi-billion dollar market!". Wouldn't tell me what it was until I joined. Above average team from internal transfers ("our hiring standards are way above Amazon's regular bar!") and external hires. I even got smugly booted from a team meeting where I had presented some code I had written, after some SDE 2 realized I hadn't yet signed the NDA or whatever.
It ended up being Kindle ads for Amazon Local, the now defunct Groupon clone. So anti-climactic after all the cloak and daggers theatrics. At least Amazon Go is a much bigger deal with interesting tech behind it.