“opt-out” is certainly correct (though, as I understand it, Pocket, as with all parts of Firefox, is only loaded when it is actually used, so “opt-out” does not seem to tell the whole story to me). I do disagree with the “non-removable” bit. Fully disabling it, to me, counts as removable (though I can understand why someone would disagree).

Perhaps I just read something into the Author's tone (probably due to all the vitriole from the typical discourse around the integration) that wasn't really there. If that's the case, and all the author meant by that statement was that the code itself could not be completely erased from Firefox as-packaged, then that seems to be factually true, and I simply read it incorrectly.