The proof is in reading the entire sentence you quoted:
In historical Church documents in Medieval times and before you will find that when referring to members of the church it almost always refers strictly to the hierarchy of the Church.
The Church's practice of withholding the cup from the laity only served to increase the distinction between the church (who practiced communion of both kinds) and the commoner (who was only permitted communion of one kind, bread).
This led Huss to argue that John 6:54 required all to be permitted communion of both kinds and also that the definition of church (ekklesia) should be as a congregation (all are the church) and not just that of the universal church (the hierarchy is the church).
Examples?
In any case, that is not what the original citation claimed, which was that the official teaching of the Church at the time was that lay people were not members.
This is utter and absolute nonsense. The sentence you highlight involves a halftruth that, in this context, becomes a total falsehood. But several earlier posters are right. This was not posted to seek truth, it was posted to inflame. It's not worth the time to explain exactly what the Church taught at the time about the nature of the hierarchy of the Church and of the laity. The article is a tissue of misrepresentations, half-truths and whole-falsehoods.