alt Hacker NewsEh? They didn't/aren't using their stockpiles to anywhere near the extent the US has! There reserves are much bigger - they have about a years supply - yet they used less in absolute terms than the US!
What they did was aggressively reduce demand by accelerating the switch to EV's and firing up a few more coal powerstations.
In terms of influencing the midterms - I suspect China isn't anywhere near the front of the queue in hoping Trump get's a beating given the chaos he is causing.
Complaining about foreign interference is ironically straight from the Hillary playbook.
They are strategically using their stockpiles. There are a lot of factors in their strategy and some of them conflict. This has an effect on the world market.
How much of their strategy is to change elections vs other concerns I don't know.