The purchase of ~hundred weapons and delivery vehicles from Russia will be helpful, because it does give China a credible 2nd strike capability. China could probably launch 20 missiles against the US in a second strike after the US devastates part of china in a pre-emptive strike, which would be enough to deter a rationale country but might not deter the US, since 20 nukes would be devastating but not necessarily civilization ending.
Russia may be willing to sell to china because of the following a), china already possesses such a capability, but just needs more weapons b) from Russia's security perspective, would not increase china's abilities to strike Russia (which it already has with short range), nor Russian ability to strike China (which is also possible with short range) c) provides an avenue to monetize a weapon system that is normally not monetizable (since its not as easy to sell missiles as jets, even though jets cost more).
The fact that China already possesses DF-41 makes a sale much more likely for Russia, since it could be treated as a commercial transaction vs broader strategic considerations. Again, the idea is not that china needs top of the line from russia, but just more bulk for the next few years, and even older stock would be sufficient. Another thing is that there is still a great disparity between China and Russia nuclear weapons stocks post launch, so it does not constrain Moscows capabilities in thier wargame scenarios against China.