As I said, it may not be the most wise thing to do with an exercise that you have been invited to attend for the first time...but there are also parts of the exercises that the PLAN is not a part of, particularly the very involved naval warfighting exercises conducted between the US and its allies. So it is also not too unexpected.That's the most important part. Is it usual for a nation participating in international exercises in which it takes part itself?
The PLAN is involved in maritime security and anti-piracy exercises with its FFG and DDG, with its AOR helping. Its hospital ship is involved in humanitarian and medical exercises. So they may want to try and gather some signals and communication intelligence on the other exercises if they can.