That's not the entire story. Google still has web.dev, which blurs the line between "web platform" features and Google-specific or "experimental" features. Neither of the two actively promote MDN either (web.dev doesn't mention it even where it would make sense).

This doesn't seem as much of an intentional decision of the two corporations to promote MDN as an independent resource but more likely the result of developers working for these companies refusing to compete with MDN.