I'm in agreement with you there.
The Vatican itself doesn’t agree with Sister Marian’s extreme viewpoint. It recognises all other Christian denominations as Christian. Sure, they may not be as old as the Catholic Church, or have as direct a lineage to the Church of the Apostles, but they’re still at least more or less Christian. This means that Marian, by her standard, is dissenting from the Catholic Church and thus is not a Christian.
Good article by TIA.