I think that is incorrect, maybe you mean "must be a citizen of the member country"?
I don't live in the Schengen Member country any more, I live in Thailand, but I have sponsored my wife every time when we (she) applied for a Visa at the Embassy / VFS and this was always accepted.
As I don't have a house there any more, I have friends guarantee (sponsor) the accommodation, and the first time my wife applied for the visa the girl interviewing her said "ooh you have 2 boyfriends" because she saw 2 guarantee (sponsor) entries ????