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I wouldn't be surprised if they were just manually adding a header.

I've seen it done like that before, using a browser extension to add a header and then mod_rewrite to apply a special set of rules if that exists.

The article specifically mentions how this special HTTP header was implemented client side using a browser extension and how it's handled server side

I doubt it. I think they've actually just made up an HTTP header.

(Cookies wouldn't work anyway... how would you place them? What about expiration? How would they interact with existing cookies? What if you have to clear your cookies while debugging? Whereas a custom header requires a browser plugin, but otherwise is innocuous.)

More likely a useragent change