"Those who attack the Church of Christ by teaching that Christ's Church is divided into so-called "branches" which differ in doctrine and way of life
Not what Eastern Orthodoxy teaches - agreed.
To be fair to the Anglo-Catholic Anglicans who believed this, they didn't think groups with fundamentally contradictory beliefs were 'branches' of the apostolic Church, but rather that all the episcopal, sacramental, liturgical churches that shared a modicum of basic orthodoxy were, with the insight that these groups, however much they may have fought in the past, shared the same basic beliefs. (They were wrong when it came to Anglicanism's place in this theory.)
The view being attacked here is really that of Protestantism.
The Catholic Church doesn't really believe in the branch theory either, which I'll explain more below.
or that the Church does not exist visibly, but will be formed in the future when all "branches" or sects or denominations, and even religions will be united into one body
A false gospel of liberal Protestantism: the relativism of the World Council of Churches and the Consultation on Church Union (which sought to merge Anglicans with Presbyterians and other Protestants).
and who do not distinguish the priesthood and mysteries of the Church from those of the heretics
Ah, but when the Catholic Church looks at the Eastern Orthodox, it doesn't see heretics, but itself. EOs never have dogmatically
declared postschism Catholics heretics. So this condemnation can't apply to an opinion
that recognizes the other side.
but say that the baptism and eucharist of heretics is effectual for salvation
Saying such are absolutely graceless is allowable hardline EO opinion but not EO dogma - I'm pretty sure ROCOR doesn't arrogate to itself a claim to dogmatize for all of EOxy. Dogmatically, all of EOxy treats all non-Orthodox sacraments as a question mark. If this condemnation is against dogmatizing that non-EO sacraments are the same as EO ones, I see the point.
IMO, there is no way the Protestant sacrament of communion can be the same as the EO or Catholic Sacrament - they deny it's wholly Him and God won't go where He's not wanted. (Sincere Anglo-Catholics do believe it's wholly Him, and in spite of their illogical position ecclesiastically I wouldn't be at all surprised if God touches down on their altars.)
A middle-way EO opinion I've learnt here and elsewhere is the belief that God does give sincere seekers some kind of grace, including through their sacraments, but that's not the same
as the grace from EO sacraments.
therefore, to those who knowingly have communion with these aforementioned heretics
I can't name one real Eastern Orthodox church that officially intercommunes with anyone else, let alone with Protestants.
or who advocate, disseminate, or defend their new heresy of Ecumenism under the pretext of brotherly love or the supposed unification of separated Christians, Anathema!"
True, but neither the Eastern Orthodox communion nor the Catholic Church formally holds to this heresy!
How can we be so sure about that? Have we not learned from the broken promises made in the creation of the Unia in Slavic lands? What one Pope of Rome says or does, another can undo just as easily.
'The creation of the Unia in Slavic lands' was 400 years ago. Since Balamand, AFAIK no new proselytism via the Eastern Catholic churches has happened.