They have $340bn of debt; even if they default on the whole amount, it's only 10% of China currency reserves.

The problem, however, is not the nominal amount but how the company network is inter-connected with other companies (either abroad or locally). This amount, albeit not colossal (by China relative size) could trigger effects in other companies that will lead to their failure. This will go on until it becomes a full international crisis.

Of course, it could mean that just some rich intercontinental people have lost money on some Chinese developer bond. The thing is, no one knows. Not the leaders of the CCP nor the US do have insights whether this can go somewhere to just stop here.