A screwed US economy doesn’t help their export market either.
Actually, in a roundabout way, it does, for a start China exports more to Europe than it does to the US, who have managed over the last 30 or 40 years to transfer their middle class spending power abroad - along with a large chunk of their jobs. Currently China's exports to the US are growing at a far slower rate than their exports to Europe - in fact it won't be long before Hong Kong overtakes the US in the Chinese imports tables.
Europe and Asia is a growing export market for China - the US is a shrinking market, it makes perfect sense to nurture a growing market, that's Capitalism 101 - speculate to accumulate.
Buggering up the US economy also definitely helps China in other ways, China's aspirations to replace the US as THE global superpower for one. A key requirement to attaining that status would be a weak US economy allowing China to push even harder to replace the dollar as the standard world currency. China has long suggested that a currency controlled by the IMF would be preferable, how does that help China? The clue to that is in the article you posted:
" China would have to get something in return for its generosity. This could be the spur to giving China a much larger say at the IMF. In fact, it might be necessary to make clear that Christine Lagarde would be the last non-Chinese head of the organization."
Of course what China would like to see once America was forced to buy commodities and trade in a new international currency would be a weak dollar, reducing the US spending power. Fortunately they couldn't conspire to make that happen, or could they? Surely to do that they'd need to hold oodles of dollars in US debt that they could swamp the market with to devalue the dollar.
Aren't conspiracy theories great?