Your logic doesn’t make sanse. I will tell you why. We had LCA Tejas in making at the time of MMRCA. The reason for rejection og Gripen was cited as India having a similar aircraft in making at that time. Here, China was flying J20 5 years ahead of they received first Su35 and 8 years before last Su35 was delivered. China purchased S 400 much later than they had HQ9 and other HQ in operation.Your logic is a "less advanced" unit is bought after more advanced unit is inducted. First you're wrong about Su-35 purchase anyway. Negotiation date, purchase date, delivery date all different. Negotiation happened around 2006. Different reasons because Su-35 was changed a few times around that era and purchase deal date was around 2010 but deal specifics were still not decided between Russia and China because Russia then was switching to BM version of Su-35 and 117 engine. This time J-20 was 8 years away from service and PLAAF had no idea really how long it would have taken CAC to complete J-20 back around 2010 when Su-35 purchase negotiation details were still being discussed.
Anyway even using your logic which is poor, import has nothing to do with this. Import or not, it is about more modern vs older. Even ignoring your timeline problem, it still is believable for China to buy su-35 even if J-20 was in service in 2010 and Su-35 bought in 2020. Even if such a thing were true, there is still good reason to do this. Just like the same reason US bought Kh-31 in the past and bought or stole all kinds of foreign equipment to test and work out how good it is and how to defeat it. Remember India has MKI which has thrust vectoring using similar layout as Su-35. Back then China did not have its own thrust vectoring which J-10 eventually shown. It uses a different system and type of twisting nozzle to Sukhoi method but in 2010 PLAAF was definitely interested to know more about it and how well it works.
China has only ever made one purchase of Su-35. The deal was done around 2010 and negotiation on details kept changing and Russia wanted China to buy more. Because China thought it was useless to have more than just a few for evaluation and testing, eventually both agreed on just 24 which China thinks is low enough to not waste money and Russia thought was enough to make money from. You think if J-20 is truly not good enough, China would only buy 24 Su-35? Maybe 240 wouldn't even be enough because already J-20 number is about half of that only three years after introduce to PLAAF.
Now the information is available and out, will let idiot sore butthurt one run his mouth. At least readers know and he can make fun of himself as much as he likes.