The exact reasons aren't entirely clear, originally they hated NS because it allowed Europe to ignore Ukraine in the gas trade which left them more exposed. By the time of the full scale war I would bet the reason was more "fuck Russia" than anything more carefully reasoned.
https://www.reuters.com/business/energy/russia-china-bless-v...
So Russia can now export gas, get foreign currency, and buy weapons with the money. I do not see any strategic wins here.
Additionally, China gets an economic boost. That is a sublime strategy.
1. Power of Syberia 1 throughput is not fully utilized.
2. China pays half of the EU price.
3. Power of Syberia 2 not be build in the near future. It's not the deal to actually do something. It's too continue further discussion.