Russia experience in designing competitive passenger liners may seem deficient next to those of airbus and Boeing, but their experience is still greater than those of the Chinese by a larger margin than theirs might be less than those of airbus and Boeing. Their last wide body passenger liner is the Il-96. Previously they have produced a number of wide and narrow body modern passenger jet liners that, while not all together competitive in efficiency, nonetheless meet all international standard. Examples include IL-86, Il-62, Tu-204, Tu-154, etc.
In the field of airliner design and production on a scale of 1 through 10, Boeing and airbus might be 9.5. Anything less than 9 is not probably not really competitive. The Russians may be 8.5. The Chinese at the moment might be 6-7. It is not a bad idea to leverage Russian experience to get to 8.5 quickly.
When did the Russia design the II-96? How good was it at the time? How much of the technologies from II-96 are still applicable today? Did Russia have any other wide-body passenger airliner design experience since? How old are the people who were involved in the II-96 project?
I don't think technologies are the primary reasons for China to form the joint-venture with Russia. You would think China has gained tremendous experiences after ARJ-21, C919, and Y-20, any experiences and relevant technologies brought by Russia on wide-body passenger aircraft, while helpful, are rather limited and are not something that China can not overcome by their own efforts. Relative to the potential risks and challenges such joint-venture could encounter, the benefits are not large enough. Someone also mentioned about financial, I think that's even less a reason than experiences and technologies.
The question then is what exactly are the reasons China decided to pursue such a path instead of going it alone? I believe SamuraiBlue touched on above one key reason: market and marketing. C929 is not C919, which has much larger demands and the Chinese domestic market alone can absorb a large number of them to make it close to commercially viable. C929, instead, is much larger and longer range, and is therefore not meant for domestic market. China needs a partner that can bring both some demand and credibility for marketing purpose. Russia is the best, realistically speaking, candidate partner out there.
The other reason is international politics. A strategic program like this one binds the two nations more closely. The trading relationship between the two countries is a bit too one-sided.
Having said the above, I feel that this partnership is fraught with risks and potential roadblocks.