I don't understand. If they're proxying requests, what would this variable ever be used for? This is at an endpoint for a client or their javascript to consume (or just as a recruiting-marketing tool). That would be redundant if they're proxying requests and insanely silly if it's the only way (instead of proxying) that they flex new features on... as they'd be visible in the javascript and it would be trivial to spoof the response to be "yes".

I have no doubt that their internal employees or a subset of them are using a different build of Facebook, and maybe I'm missing something, but I don't understand how this is related.