FWIW, the API they provide here is a subset of the API provided by a local Ethereum node, and also the same general API surface (with the exact same protocol) as the APIs provided by Infura, AlchemyAPI, and myEtherWallet. While I also went looking for a testnet endpoint from Cloudflare (and didn't find one), it is arguably superfluous as it is drop-in compatible for things that do have testnet endpoints.