Video is incorrect. Russian is South Slavic language. It's much closer to language used in Montenegro (which is South Slavic) than to Ukrainian or Belorusian languages. Russians have troubles with understanding of something written or spoken in East or West Slavic languages, while they have much less trouble with South Slavic languages. It's because Russian language is based on Church Slavonic language[1][2], which was develop by Saints Cyril and Methodius[3] using South Slavonic dialect spoken in (now) North Greece.
Also, samples of history are incorrect, causing multiple LOL's.
[1]: https://en.wikipedia.org/wiki/Saints_Cyril_and_Methodius