As to your edit, I absolutely agree. That's what I meant when I said pushing the add-on was not good. I was only suggesting that an add-on is a reasonable place to implement an easter egg, since it's separate from the core browser code. Distributing that add-on is a different matter, and I personally disagree with what happened there.
It was not a common add-on but an ad disguised as a "study." The question is still: why?
Why did that marketing team need that treatment instead of giving to the interested users a link to the normal add on? What was the actually planned scenario? Was it planned that that "study" (the studies are apparently officially "a way of making more informed product decisions based on actual user needs") uses some functionality not available to the normal add-ons? Was it that the normal add-ons wouldn't have access to the API that the "study" would use but that is forbidden to the normal add-ons since v57?
"The addon is actually deployed as an embedded WebExtension, which is subtly different. It has a 40-line legacy XUL/XPCOM bootstrapper controlling whether the WebExtension part of it runs. The legacy code actually could upload your hard drive and isn't bound by any of the WebExtension restrictions. We know it doesn't do anything harmful, but it could have done so.
The WebExtension itself also has <all_urls> and webRequest permissions, granting it the ability to sniff the content and headers of every page."
(Source: https://www.reddit.com/r/talesfromtechsupport/comments/7k7wu... )
The question is still: what was the goal?