This is LEO, so it's everyone's orbits. And in any case the big worry about space junk is not so much that it takes out one or one thousand existing satellites - which China could replace - but that it makes the orbits unusable by future spacecraft.
The point is that having junk in your own orbits is an interesting self goal. It opens the window to hybrid war strikes, for example.
It takes a lot of energy to change orbital planes. Debris tend to stay in constrained orbits (usually their original ones). If the upper stages are in the birds’ orbits (which is a big if), the debris will all tend to stay there.
> they have the capabilities to do it in a more targeted manner
But not plausibly deniable. My point is China is leaving the front door open to shenanigans by leaving high-energy mass next to its birds. (If, again, it is.)
I don't see any plausibly deniable scenario involving apparent spent rocket stages suddenly reanimating in militarily useful way. c.f. routine electronic warfare jamming. Even a "malfunctioning satellite" would have more deniability, and certainly equal ability to threaten others' space assets.
"Never attribute to malice that which is adequately explained by stupidity."