Fortune 100 companies are definitely aware that anything in email is evidence. I doubt anyone wants to admit negligence or anything. And by offering you some big thanks they only validate that they were real passwords and not some junk data.

For an enormous corp to take down a file in a couple of weeks is thanks and recognition enough I think.

You might have gotten some admin or mid-manager. Maybe they wanted to cut off communications (pretend it never happened) before their boss finds out about it. Seen guys do that before..