Avira,
I was going by a secondary source that, upon further reading, might not be fully unbiased. Looking for hard evidence, I see only one specific reference that is obscure, but meaning seems to be clear.
letter to Joseph Apfel, 1956 – suggesting not to translate Frankel’s Darkei Hamishnah: I am afraid that zealots in England will attack you because they consider him min and apikoires because R Shimshon Rafael Hirsh battled against him.