There's no evidence. We had a member here who said he knew the guy that found the casings. The guy turned it over the "the authorities". Who "they" were the guy didn't know. There are no casings to be examined, so in reality there is no evidence that it ever happened. For all we know the guy made it up, or found some 5.45 that some guy was blasting out of his SAR-2 in the middle of nowhere.
The bottom line is there is no proof of any of it. None.
Now, you really have to ask yourself a couple questions about this.
1) Why would you bring a weapon that is easily identified as your's to a place you shouldn't be? Would it not make more sense to use a couple of the several million 7.62x39 caliber ones that they already own for a "spec ops" mission on the border?
2) If you want to look at the border, what would you do? Yeah, you'd just go down there and look at it. It's not like anyone's guarding the damn thing. So if you were China, why would you try some bizarre "spec ops" operation, armed with unique weapons that could only be traced to China, when any of the thousands of spys you already have here in the USA could just drive down there and look all they want.
Sorry, it doesn't pass the smell test. It simply never happened.