Not really true; they were principally against zionism until they thought it could benefit their national interest
Lenin was against Zionism from the get go
Stalin was against Zionism up until 1944 iirc (this definitely had to do with his partnership with UK and USA)
USSR became officially pro-zionist in 1947, but this didn't last long, only until the mid-50s when they reverted back to anti-zionism
the ussr "quickly" found out the state of Israel wasn't going to serve their interests at all, so subsequent leaders were all against Israel up until Gorbachev who the PLO accused of trying to please zionists