SinoSoldier
Colonel
I don't think China accused it of being against international law, but against international practice.
In this case, China was in contact with the US and had confirmed publicly that it was theirs, so normal practice could have been to discuss a means to secure it and minimize damage to anyone.
Instead, presenting it as a threat by both government and media, and then destroying it as if it was an enemy to be vanquished, can reasonably be described as an "overreaction" which is the term the Chinese government used as well.
As far as we know, the balloon shot down by the J-10 was not claimed by any other party and as far as we know no one was in contact with the Chinese government about it to claim that it was theirs.
So yes, somewhat different.
The balloon is almost certainly a SIGINT platform, so every second it is left in the air is time that the US is allowing it to gather more critical data, hence the urgency to shoot it down. Destroying it is necessary as a defensive measure.
And I don't think the US needs to justify shooting it down despite the military necessity to do so. It is an adversarial military platform operating in the US' national airspace for crying out loud.