So what would China's motivation be here?
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Russia is desperately trying to make the China-Russia thing a reality, and is probably trying to drag them in against their great resistance. China has zero credible reason to be dragged into Russia's nonsense, and a billion reasons why they want nothing to do with it.
The ideal outcome of this is that China realizes that Russia is outright trying to drag them into conflict, and that they repudiate that country entirely.
The Russians could have done this with a fishing trawler (they cut cables accidentally all the time), so like you I doubt we can infer some nefarious Chinese plot from the flag on the vessel.