I don’t think changing the RPC URL exec’s an ethereum light client.

Extending the mail server analogy, updating the MX record does not exec a mail server.

He's saying that configuring MetaMask to work with a light client is as easy as changing the RPC URL. The light client still needs to be installed/configured separately.

Ideally, as Moxie suggested, MetaMask itself integrates a light client into its wallet, so that it becomes the default configuration.