I disagree. This is the classic case for protocols. Look what he says:

"I’m using a protocol for the Account type. There are classes that conform to the Account protocol: TwitterAccount, AppNetAccount, LinkedInAccount, etc."

The important bit is the "etc." at the end. The types of accounts he wants to support is open ended. He also mentions a plug-in API. You can't have third parties come in and extend some algebraic data type.

If this is not a case for protocols then there is no such case at all and protocols should be removed from the language.