What payload would possibly be worth this, that wouldn't be more effectively served by a different delivery mechanism?

a plausibly deniable payload purpose

Any 'useful' payload would erase any plausible deniability. If the idea is to merely drop a piece of scrap metal on some politicians house or something, with no warhead involved, I don't think they can do that. Reentry is too chaotic for a piece of unguided rocket debris to hit a target that exactly.

How is "Chinese space junk falls on important target" plausibly deniable? Especially compared to alternatives that conceal source?

It sounds good, but I'm struggling to think of a use case for (1) an important enough target to be impactful, (2) that could be effected by space debris, (3) for which there's a distinction between "we accidentally hit it" vs "we deliberately hit it"

I don't think China cares. At all.

But in this case there are plenty of other, better delivery mechanisms than disguising the payload as space junk.

Make it land on a valuable target and not make it looks suspicious?

You're forgetting about what happens when something "falls" from the atmosphere uncontrolled

But what if it only appears uncontrolled?

People who know how to track this stuff knows if it's uncontrolled or not