Rating of debt is largely a question of future ability to repay. The concerns of the rating agency, I imagine, is that much of the debt in China is being created for non-economic investment, that ultimately won't be able to pay for itself. And someone will have to foot the bill, whether it's the lender (via a write-down or write-off of nonperforming loans) or someone else (the central government, by assuming the loans directly or some other mechanism).
Yes, the Chinese central government has a lot of capital to play with, due to a very high household savings rate and positive trade imbalance. But the magnitude of the internal debt, and the degree to which it's likely misallocated and ultimately nonperforming, should concern anyone who's paying attention.