Perhaps because the RD-93 was considered low risk & low impact on strategic relations by the Russians. Su-35 is whole different game. It'll surpass the Su-30MKI. You think PRC will do a double cross? RD-93 had no issues. It was going to the FC-1/JF-17 and that was clearly/officially known. You think PRC is going to lie and re-sell the Su-35 and the Russians (and Indians) are stupid not to know that?
Su-35 is not going to Pakistan or North Korea.
The idea of low risk or low impact is very subjective. I would view RD-93 engine has a bigger impact than 24 Su-35 if provided to PAF.
If PAF equipped with 200 JF-17 all running on Russian RD-93. It will almost fully modernised bulk of its Air Force replacing many obsolete aircraft like Mirage III, F-7PG, Q-5 Fantan. But if Russian refuse the re-export, modernise bulk of PAF will failed in next decades because PAF do not have so much hard cash to replace bulk of its ageing aircraft. JF-17 provide the most idea chance to modernise bulk of its AF without stretching its finance. The only draw back is engine as Russian engine is the only option. Europe and American engine is out of question. Chinese engine will need time and more finance from Pakistan to hasty the project.
The 24 Su-35 if provided to PAF impact will not be as great as 200 RD-93 engine provide to PAF.
Plus, Russian is desperate to get its first export for Su-35. If sukhoi needs to be successful or financial healthy. The interim solution is to get Su-35 export sales. With failed bid in South America, India. I am sure Russia will do whatever it can to facilitate an export sales.
Russian used to refuse PAF for its Sukhoi Su-27 but now is a different situation. India has shift more towards Western for arm which is irking Russia. Of course Russian know how to play its card. RD-93 engine re export is a good start and show change of attitude from Russian toward Pakistan.
Of course Russian still has many military dealing with India but Russia needs to think of its pocket for future expanding its military hardware export since China and India source are drying up soon.
China do not need to lie for Pakistan. If can easily cook up an excuse of Su-35 not suitable for its own need and decide to re export it or whatever.