He was deprived (if indeed that is possible) of membership in The Chapter; not The Catholic Church, as I read it.
I looked up the canon cited on vatican.va ...
Can. 1373 A person who publicly incites among subjects animosities or hatred against the Apostolic See or an ordinary because of some act of power or ecclesiastical ministry or provokes subjects to disobey them is to be punished by an interdict or other just penalties.
So yes, he is being deprived of attending the Chapter, nothing said about being deprived of membership in the Church.
Although that's not to say his enemies would love for that to be the case.
A couple of things: First, this canon applies to an Ordinary, not the head of a Pius Union, which is what the Society was erected as canonically. Thus, this is no proof of anything except perhaps delusions of grandeur.
Second, Pope Benedict already said that Bishop Williamson never was part of the "greater"Church, and thus never converted to Catholicism, only to the Society. Thus, in fact, the Pope is saying that he is not a member. Now, of course, this is rubbish since Bishop Williamson converted not in the Society, but was received by a priest who was not even a traditionalist. This shows that the Pope is either woefully unread on the matter, or that he is being less than truthful.