Didn't Russia originally refuse to sell AL-41F engines to China? They had concerns that China was only interested in reverse-engineering those engines. Allegedly, they agreed to provide the engines only if it was part of a large Su-35 order. The whole transaction felt more like an indirect technology transfer. It seemed like China bought all those Su-35 just so their engineers can take a peek at the AL-41F.
Maybe this flirtation with the IL-78MK-90A was the same thing. Didn't Russia also refuse to sell PS-90A engines to China? Once the WS-20 engines entered service, the Russians may have become less guarded about letting Chinese engineers study the PS-90A engine. But the Russians would only agree to provide the engine if it was part of a large IL-78MK-90A order, just like they did with the AL-41F and Su-35.
I can't think of any plausible reason why China would be ordering Russian IL-78 fuel tankers.