While Fr. Seraphim Rose did not view the MP's compromise with the Soviet Union as favorable, he always believed that the Moscow Patriarchate still possessed grace. The situation for the Church under Communism isn't as black-and-white as some in Old Calendarism wish it had been, and there were men and women all the way up through the 1980s who were martyred for their faith, even though they belonged to the Moscow Patriarchate.
Fr. Seraphim Rose's recognition of the complexity of the situation definitely does not make him a heretic. It's worth noting that he defends himself against those who accused him of being a "Cyprianite" in the book Letters from Fr. Seraphim, where he argues in a letter that those who accuse him and ROCOR of being Cyprianite are infected with what he called the "correctness disease"; a disease which would cause them to eventually leave ROCOR and the Orthodox Church.
At the time those making these arguments were under ROCOR, but they would leave the Church and eventually form the infamous "Holy Orthodox Church in North America", the home-grown "synod" of Holy Transfiguration Monastery in Boston, MA. Fr. Seraphim's diagnosis tragically proved to be on-point.