> junk being in everyone's orbits
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.)
Orbits can intersect and naturally decay, at different rates according to different drag coefficients, that's why large pieces of debris require periodic avoidance manoeuvres as they cross others' paths, and the less predictable movements of debris from an actual collision would be a nightmare for years. Which means your space warfare strategy probably doesn't involve causing collisions unless you've contingency planned for losing your own assets.
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.