Unencrypted, client legible access tokens is probably one of the worst ecosystem mistakes you can make. It lets devs (like the article author) read oauth as oauthN "because I can read the token". So now you have clients taking a silent dependency in your ecosystem on being able to predictably read the access token, even though they're not the audience. Suddenly, decisions like "change the format of the token for this resource" or "let's start encrypting" become ecosystem shattering projects.
Encrypt your access tokens by default if you want to guarantee clever clients don't try to take a dependency on them.
It is a conflation of the access token (message to the protected resources about the client) and the identity token (message to the client about the user).
OAuth doesn't define the concept of an identity token, hence it not being usable itself for authentication OpenID Connect extends OAuth with an identity token, which is of course readable by the client.
To be fair, Facebook Connect explicitly made this choice to combine the identity token into the access token - so their access token is necessarily client-readable and verifiable.
Encrypt your access tokens by default if you want to guarantee clever clients don't try to take a dependency on them.