I'm personally surprised anyone expected it another way.
Hi/low mix never really worked with twin engined type duos.
Su27 and Mig29, F14 and F18, just to list a few examples OTTOH.
The J31 had no PLAAF future the moment they chose CAC's design over SAC's offering for the XXJ/J20.
SAC was being stubborn and arrogant in thinking they could change the PLAAF's mind with their re-hashed design and political maneuvering.
If anything, I would expect it to have been their behind-the-scenes maneuvering that would have got the PLAAF's backs up the most and made them dig their heels in to resist the J31.
Had SAC put the kind of effort they did into aircraft making and designing they had into political intrigue, they may well have themselves a military contract and order for the J31.
I have a feeling the PLAAF top brass has just about had enough of SAC's under performing nonsense, and seems to be determined to give it a very public and high profile disgrace. Which is highly unusual for China.
Normally, you would expect the looser to be thrown a face saving consolation prize, rather than be so thoroughly routed and disgraced publically.
I am guessing the PLAAF is being almost deliberately harsh so as to raise a giant red flag to the powers that be, in the hopes they will take a long hard look at SAC and give it a thorough purge to rid it of its toxic and incompetent leadership elements and their political backers.