My guess is they implemented dark mode, then discovered some legacy posts have videos with transparent backgrounds. As a quick fix, they decided to disable dark mode anytime someone scrolls down to a video.
Seems like one of those compromises to solve an 11th-hour bug.
The contents of this article are pretty old, but the static website's design has been revamped (I believe several times) since then. My guess it that the two may have just fallen out of sync in such a way that this particular oddity manifests.
