As recent news has reported China has a French nuclear power plant. The only way France would be able to sell to China and China be able to buy it from France is if both are signatories. That's how they control who gets nuclear power technology because one of the rules is allowing outside inspectors in to count how much each country has i.e. how much uranium. Yes if a country has a domestic supply it would be harder to track. But for countries that don't have a domestic supply, inspectors can and do track accountable supplies. If some is missing from the last inspection, then they know something has been done and are subject to action.
This dates back to the 1980s where programs from the CCP reveal (and in subsequent years, proved) China's wish to study, evaluate and adopt commercial nuclear powerplant technologies (these things include entire sets of safe operation procedures and operating standards) along with the best available commercial powerplant designs that are offered by the Russians and French.
China's domestic design on commercial powerplants were divided into deuterium isotope reactor (heavy water PWR type), thorium reactor research, molten salt reactors, and "conventional" pressurised water reactors. Even Westinghouse design was bought for 3rd gen design. The final result of evaluating and learning Russia, American, and French reactor designs culminated in China's own CPR1000 as it's first step domesticating and combining the best of those technologies as it's own 2nd gen commercial reactor and then the Hualong 1 design as a 3rd gen reactor design. France and Russia have both continued with their own designs and China have continued buying them. It is one way to buy lessons when you are behind in these fields... particularly in the subfield of operating them.
This has little to do with weapons supply chain. Fissile material that are accounted for in energy production are indeed well accounted by international auditors. Like you said, it is common for all signatories of any nuclear energy agreement. What isn't within the scope of treaties is domestic fissile material mining or refinement. China has been able to do that since the 1960s.
Any fissile material that remain off the books in accounting for energy production, stay off the books the entire time it is in the ground, in the shop, in refinement, in a warhead. There is no mechanism for accounting anyone's (that includes the US and Russia) weapons material refinement facilities and the volume of the material itself.
Some commercial and academic reactors may be capable of producing the required material which can be used for weapons after refinement. There may be agreed rules on how much is allowed to be used for weapons and how they are tracked but this is always going to be the tip of the iceberg. It is a minimum of the available material the country has for refining and weaponising. Since we don't know how many facilities in China exist just for producing material suitable for refining and weaponising along with how long they've been operating, there is no accurate way of estimating China's weapons grade material based on commercial energy producing reactors and the accounting of material going to and leaving those facilities.