No, I don't mean that. What I mean is that heresy of its own nature separates one from the church. But let's say a bishop is secretly a heretic. Although he himself can't still be in the Church, the mysteries he performs can still be saving for the faithful who receive them at his hands, because, as you rightly note, "ecclesia suplet". The grace of the mysteries is Christ's and the Church's, not the personal property of the heretical bishop. God condescends to the ignorance of the faithful and continues to provide them with sanctifying grace.
But if this bishop starts making his heresy public, then the people no longer have the excuse of ignorance. To the extent they are aware of the bishop's heresy, they will receive the mysteries from him to their condemnation. This implies, however, that the mysteries are still lawful, since as far as I know you can only receive to your condemnation when the mysteries are true mysteries, i.e. you are being condemned by your own sin, not by the gracelessness of what you are receiving. What I don't really know is what it moves beyond that, and the bishop is completely deprived of even the grace to perform lawful mysteries. I.e. when does the Eucharist he offers cease to be the Body and Blood of Christ. We know that mysteries can't be performed outside the Church, and we know that heretics are outside the Church, but we also believe God condescends to our weakness and ignorance and often supplies what is lacking. So you can say that the heretics, though outside the Church, and hence by right deprived of the grace to perform valid mysteries, may nevertheless continue to exercise this power so long as God sees fit for him to do so, for the sake of the people. But because this is a matter of condescension, not strictness, it is necessarily uncertain, up until the point that the Church declares with a clear voice that a heresy has arisen and that those who preach this heresy are cut off.
The Cyprianites believes that the only body that can claim to represent the "Church" is this so-called Unifying Council. If they were simply arguing that the council had to be Ecumenical or Pan-Orthodox, and not just local, I don't think their views would be heretical, just ahistorical. But the whole point of the Unifying Council is that both heretics and Orthodox participate. I.e. the voice of the Church must include heretics! Now, some councils did allow heretics to participate, but I don't think there is any suggestion that the presence of the heretics was in any way required or that they were considered still members of the Church. Rather, they were allowed to participate for the sake of economy, in order to facilitate the return of repentant heretics, and to remove any possible excuse for the unrepentant ones. But the Cyprianites interpret this to mean that the heretics before their condemnation were still in the Church. This at the very least violates Apostolic Canon 46, and may be understood as heresy itself, since it teaches that the Church can contain heretics, despite "One Church, One Faith, One Baptism".
The other thing is that they refuse to call the ecumenists heretics, even though they know ecumenism is a heresy and even though they refrain from communion with the ecumenists. It is hard not to interpret this behavior as confirming their heretical ecclesiology.