Not to be a conspiracy theorist, but the phrase "So long, and thanks for everything" used in the tweet reminds me of "So long, and thanks for all the fish" from the dolphins in The Hitchhiker's Guide To The Galaxy. The background there is that dolphins are secretly more intelligent than humans, and are leaving Earth without them when its destruction is imminent (something the humans don't see coming).

I did once leave a company with a phrase just like that :P A few people there actually got the reference and congratulated me for the burn.