The deal is NOT sealed unless both side confirms it. So far there is no words coming out of China.
But I'll play the devil's advocate here. What would China do with them? Only Russian weapons work on the planes, so what weapon that Russia have that is so much superior than what China have so that prompt them to purchase Su-35.
Logistics, how will China service this planes, can it do it by themselves or do they have to keep purchasing from Russia to service it.
And lastly, just what technology that SU-35 have, which China is severely lacking behind, that they can learn from it?
I don't think there is much China can learn from the radar or composite material or electronics, and probably NOT Engine as well, 117S may be superior than WS-10, but those two are totally different engines from ground up I doubt they will help WS-10 development. But what other areas can China possibly learn?
Or would this be a extortion from Russia? That if you purchase my SU-35 just this once, I'll forever drop my loyalty claim from your SU-27 copies like I did with your J-7s?