You missed his point. St. Athanasius was speaking to the flock who fled the heresy of the diocesan churches and bishops. He was teaching that the "church" is those who adhere to the Traditional Faith, regardless of location of worship.
I don't think he said that at all, and he certainly didn't practice that. That is the protestant frame of mind. (the church is within you, and is wherever you are) For Catholics, the Church is where the Pope is.
St. Athanasius was reminding people to hold onto their faith even though the Church was not. We can all do that. I know that's what I do. I assist at the indult Mass, and receive Communion that has been confected by a legal Priest. The Priests politics, state of his soul, and theology is his problem, not mine.