It looks like their Firefox addon has (https://github.com/Pocket/pocket-ff-addon), and I guess by definition you can get the source of their Chrome addon and getpocket.com, but I don't think they meet the definition of open source unless they're somewhere other than Github. Also their mobile apps seem to be closed source too.
Yeah, but there's for example also a Pocket Android app and iOS app. Those should get open-sourced now, too, and I heavily doubt that they were before.
Submitting links to Pocket is open source and has always been well documented, but other client-side portions of Pocket have always been closed, such as the Article View API. [1]
It would be great to have access to article view to build other clients, such as a Linux client.
If you mean the server-side then fair enough, maybe this acquisition will help with that.