But the same guy said WS10B's thrust is around 13.2t which is not better than its Russian counterparts. They bought enough AL31 series to power both J20 and J10x.Why do they need WS10B at this moment?
The WS-15 is predicted by the Liyang employee to be ready no sooner than 2022, about the same time the next-gen medium-thrust engine is ready, but Chinese engine programs usually get delayed so an estimate of 2025 is possible. From now til 2022, that's five years, and eight years if assuming 2025. That's a solid chunk of time for WS-10B to be mounted as interim engines.
If J-20 production remains steady at ~10 aircraft per year, that's 50-80 J-20s that require engines until WS-15 is ready. Employing WS-10B for these initial batches seems like a fine idea.