Since the rest of the site uses the standard MediaWiki theme, I would have expected something that looks similar to the page banners used on Wikipedia (cleanup needed, requested deletion, etc.).
At first I didn't think there was any good reason not to notice their banner, but the more I look at it, it does look a lot like a banner ad. Especially since it goes wider than the text of the page itself.
Note that Wikipedia refers to these as messages[1], not banners, which I feel helps explain the difference in how they're perceived - whereas banners are generally decorative, with contrasting design style, wikipedia message boxes use the same design language as the rest of the article's infoboxes (solid backgrounds, don't span the full content width, thin+dark border with an image or single thick colored side for emphasis)
At first I didn't think there was any good reason not to notice their banner, but the more I look at it, it does look a lot like a banner ad. Especially since it goes wider than the text of the page itself.