You could see the existence of the extension because of poor decisions.
But it didn't turn on unless you went into about:config and manually flipped "extensions.pug.lookingglass" to true.
Definitely intended as an easter egg. Even of the people getting upset about the visible extension, 99+% of them never saw the enabled effect.
The analogy applies to computing in 2 ways: they're hidden, and it's something desirable. Niether of those ways apply to what Mozilla did. Therefore it wasn't an easter egg and you trying to pretend otherwise is just confusing.
They made a mistake in deployment that turned it into a scary mysterious blob. That's bad, but doesn't change the underlying nature of the code.
We can agree that the exact same code in C, not showing up in any lists or menus, not triggering without a magic about:config phrase, would be a clear easter egg, right?
(Also easter eggs don't inherently have to be fun, sometimes they're just someone's initials.)