J-16 plays the attack role like the Su-30MKK for the PLAAF. Need for the Su-35 depends on how urgently PLAAF need replacements. J-11D is supposed to be the Su-35 equivalent but we only seen 2x prototypes so far. So if SAC is slow or having trouble with the J-11D then Su-35 would be needed to fill the gap.
According to the rumor, China only ordered 24 Su-35. I don't think that's enough to fill in the gap. And it would take many years to complete the order. I don't see how the PLA might get the Su-35 sooner than their own J-16. Even sooner, the gap may not be big enough to worth the money.
The PLA has invested so much money and time in the J-16. I don't think they would have approved the plan to go with the J-16, if they had any doubt on the design. The Su-35 already existed back then. They could just order some. They would have absolutely no problem getting their hands on some Su-35. I don't think they would put the J-16 on hold and switch to the Su-35 at this point.
China is not at war with anyone and is not in any hurry at all for any new weapons system. They take their sweet time and do things in a very methodical manner. We can see this attitude in the future CV-17. Instead of jumping to a new system, they opted to build another Liaoning.
If they have the patience for their humongous CV program, which means they are willing to wait almost another decade for their next gen CV, why are they in such hurry for the J-16?
The most important and urgent goal for China and the PLA is technological independence. They have opted to build almost everything on their own. Even at the beginning when they lagged so much behind, they always wanted ToT to go along with their weapons purchases. That means, even at the very beginning, their goal had always been technological independence. Why would they now go for a complete foreign system, especially when this system is another derivative of the Sukhoi family (something they are very familiar with)?
This purchase makes no sense in terms of actual military and technology values. IF such deal exists, it would have been because of something else.
I'm not arguing about the technological prowess of the Su-35. It is clearly more advanced than China's current Sukhoi planes. What I am arguing is China's strategic goals: technological independence, which eventually leads to technological dominance. The potential Su-35 purchase simply does not fit this goal.