The Chinese system is very different than the West. The high hour FO was likely a check airman in the right seat and doing either line training or an annual line check. The young observer FO is just in the jump seat to gain experience. A very common practice in Chinese airlines.
Typically you fly with 3 pilots so the young FOs can gain experience during their ab- initio training style program. They have 500 hours just of observation flights before they are fully checked and have about 1000 hours actual right seat time before they are signed off to operate as a solo FO.
Many times I flew as an Expat Captain on a 4 day trip and the two FOs would trade off each day of the 4 day rotation. You may think 31,000 hours is high and it it certainly is but most Chinese pilots are flying about 900 hours per year except the past 2 years of Covid, that is the norm. Pay is also based on flight time so they also want to fly that for max pay. The more senior pilots do get the more efficient trips also so less work days to get that 900.
The junior pilots fly 14 hour duty days and 4 sectors but only get 4 hours of actual flight time and pay credit. It was said the right seat may have been downgraded and that is also very possible if he had a serious under their standards mistake like a terrain warning GPWS callout. It’s a massive punishment culture and everyone gets punished for one guys mistake. They strangle many chickens to scare the monkey in China.
I flew 8 years in China and can say it was both the best of times and hardest of times. Unless you’ve experienced extreme scrutiny and evaluation of your every input you won’t know what I’m talking about but some of the flying was actually very enjoyable and some great Copilots I flew with, much better pilots than myself down to 500 feet, you just had to pay attention or they could get you into trouble easily with a hard landing 1.7 G or above.
The simulator training is by far the most intense you’ll ever get in the industry and runaway trim, rudder hardcover and jammed elevators and stabilizers are routinely practiced in the sim but no actual emergencies can be practiced in the aircraft.