It took some time for me to find this article again. This proves that the PRC must have used WS-18 on some later batches of H-6Ks.

https://tiananmenstremendousachievements.wordpress.com/2016/04/22/chinas-y-20-to-be-commissioned-this-year-using-homegrown-engines/

According to an article on Chinese website mil.news.sina.com.cn, China’s homegrown large transport Y-20 will begin commission this year and it will use China’s home-grown WS18 turbofan engines.

The article says that both Y-20 and H-6K had to use imported Russian D-30KP2 engines, but China has so far only imported 239 D-30KP2s. 41 of them have been used as spare engines for China’s 41 Il-76s with only 198 left, of which 24 have been used for Y-20 prototypes, the known number of which is 6.

The 176 left are not enough for the production of H-6Ks, the known number of which is at least 120 that need 240 D-30KP2s. As China has not purchased or ordered any more D-30KP2 and as WS series engines have been proved usable, China must have used WS-18 on the later batches of H-6Ks it has produced.

After all WS-18 has 13.2 ton thrust, bigger than D-30KP2. Moreover, we have seen photos of a Y-20 prototype that uses WS-18s and of one Il-76 that uses 4 WS-18s.

China needs Y-20 urgently as no large transport aircrafts are available in the international market. It needs Y-20 now to carry its heavy tanks and equipment for its airborne troops and lots of Y-20 aerial refueling tankers for its large number of H-6K strategic bombers.

Source: mil.news.sina.com.cn “Depth Column: Y-20 to be commissioned this year, using homegrown WS18 engine: H-6K has already switched to using WS18s” (summary by Chan Kai Yee based on the report in Chinese)