It may have been a firefighting drill. Not impossible, some previous firefighting drill of the PLAN have been quite smoky. Of course, the drill thing is being claimed by the Chinese. We should find out the truth somewhat soon.
Duterte flip-flops a lot regarding China and the West, depending on whether the West harping on him for human rights abuses or China pouncing on the South China Sea is a stronger factor. I don't know which is stronger right now.
China tests every single new US President that comes to office with an international incident - remember that Chinese J-8 which collided with an EP-3E and the US plane ended up landing in Hainan? That was China extending the welcome mat to Bush Jr. This is China testing how much they can push...