This [0] is the tweet the article is referring to, since it doesn't actually link to it. I cannot find any official BBC source confirming this - anyone able to help out?
The official announcement [1] from NEXUS about their medium wave service clarifies they they have switched on a separate transmitter on 1323 kHz, in addition to their regular shortwave broadcasts on 7290 kHz.
[0] https://www.bbc.co.uk/mediacentre/2022/millions-of-russians-...