We'd have to start with the fact that the soviet school was adequate, but certainly not the best in defencemen development. The soviet league had brilliant forwards the hockey world does not know much about, because they never made the national team and were "just" stars on some weaker soviet league teams. Defencemen on the other hand we've always had just enough for the national team, but I would not say there were many more world class defencemen. Maybe, it's just a thought, it was because originally hockey in Russia started with bandy players. Many other qualities typical for russian players can be traced back to that. Great skating(which is falling off the cliff lately), stamina(as bandy is played on a huge surface without line changes), cohesion(as it is closer to soccer due to the large surface with more systemic player positions, so they need to know exactly what the whole team is playing and where to go to be there for the teammates). While those things are great, obviously defence in hockey is a way different animal. There is full scale physical contact, there is board battles and so on. So great soviet defencemen were always some sort of special cases and not a product of a pipeline. This heritage probably endures to this day. Btw it was the same with goalies. One could argue inviting some finnish goalie coaches to Russia when the RSL and then KHL started helped. Remember when finnish goalies were all the rage? At least I have already saw a finnish article with Finns arguing that Russia drained them and now they fall behind in goalie development
With defencemen it is the special case scenario. Every once in a while there is just a huge talent you can't really break. Then there is Romanov who is his gradpa's grandson. And he was great soviet defeneman and a coaching legend in the russian leagues. I bet he helped his grandson along the way at all stages and does it now as he is not intrested in coaching for now. There is Provorov who went to the US system at 14 and is basically a product of the US defencemen development system.
The fact remains that there is just from the grassroots on no proper development for defencemen in place. And the KHL btw is most of the time lead by foreign defencemen. So on every level there is no great environment for a defnceman to learn and/or compete. Russia surely needs to change that. Maybe with the most recently installed programs and facilities there will be some improvement there, but it will need time.
As for Devils' prospects I have to say none of them were exactly top rated D prospects, so it's not a revelation they do not pan out at a great rate. As it should be obvious from the above there are actually few russian top prospects on the D for which I would be surprised if they did not become solid NHLers at least.
As for Mukhamadullin, I would not plan the parade just yet either. While he is 18 and those are rarely getting KHL action. He is a former forward. See? Special case. It might work. I just would not put him in the same basket as Provorov or Sergachyov though.