As I get it, no one is actually believing in Bitcoin here
And all that noise that people were making nonstop 24/7 about Bitcoin being "digital gold", "the best store of value ever", "world money", it was all thorough bullshit and elaborate lie, right? So is there a soothing light at the end of the tunnel or is it just the freight train coming our way? On the other hand, Russia not doing well financially (with that freight train coming their way) may be desperately looking for a way out ("wunderwaffe"), and in that case we can expect many strange decisions not quite in line with sound reasoning and logic, bordering on insane
I don't think it's a lack of belief, it's more of believing that Russia will not be able to afford to risk that amount for long-term gains. A lot of believers here are in for the long haul so they couldn't care less about how well Bitcoin is doing in the short term -- something that Russia should probably be very concerned about
But the same question may be asked why they are buying gold then
Russia has been actively buying gold for over a decade by now, since 2006 to be exact:

And given that the gold market is not every elastic overall, i.e. you can't sell industrial-scale amounts of gold without crashing the prices (as something like that had probably happened in 2008), so it is not something that Russia should care about in the short term. Personally, I'm inclined to think that it is just a waste of money anyway as gold won't help the Russian government even in the long-term either. But from this perspective, Bitcoin is not much different from gold (likely just as useless)