Corney...I'm not defending the Russian position, though I understand it. The particulars of why the religion was deemed extremist included its claim to exclusivity, its refusing blood transfusions and the supernormal control over citizens private lives.
According to a Federal law passed on July 25, 2002, No. 114-FZ “On Combating Extremist Activity”, extremism is defined as “propaganda of exclusivity, superiority or inferiority of a person on the basis of their religious affiliation or attitude toward religion.”
And from from: https://tass.com/world/939781
"Checks have found that the organization is in breach of the law on resistance to extremism. In particular, the organization’s religious literature forbids blood transfusion for its members in defiance of the doctors’ recommendation," the spokeswoman said, providing documentary evidence about one such case.
....She said top church officials purported to enforce church rules "but in real fact the talk is about total control of an individual’s personal life — his intimate life, education and work."
Religion News Service:
Patriarch Kirill, the head of the powerful Russian Orthodox Church, has not commented on the crackdown, but senior officials in the church, a key Kremlin ally, have been enthusiastic in their support. The Witnesses, said Metropolitan Hilarion, an aide to the patriarch, “manipulate people’s senses and destroy minds and families.”