> Are they in the same orbit as the satellites? If so, China is effectively mining their own constellations
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.
> 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.
The junk being in everyone's orbits means it isn't particularly useful for the purposes of warfare though. If major spacefaring powers want to take out adversary satellites they have the capabilities to do it in a more targeted manner to various levels of deniability without background junk adding anything to the equation.
> 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.
I think this one fits here:
"Never attribute to malice that which is adequately explained by stupidity."
In rocket science, is the bar for stupidity really really high, or really really low?
Sounds like an excuse for how stupidly the Trump administration is acting, but it's actually an extremely toxic combination of malice plus stupidity. You don't have to be smart to be malicious. Occam's razor says "why not both?"