Aha, got it! Here's my sophomoric thinking—RAD is what I'd expect in most human endeavors, from Pokémon to the Grand Canal, and I'd expect to see it in abundance on Wikia. But nation-grade Wikipedias seem…different. I'd expect a healthy Wikipedia to have linear growth as it (1) creates new articles and (2) freshens old ones. RAD there means the speakers of the language don't care about the Wiki principle/goal (& there are interesting reasons why that might happen).