This seems like an overly abstract definition of "growth".

Maybe so. I don't see what else growth could mean in real terms other than an increase in economic activity and production. As I said, under that definition, there's no way growth will decrease under socialism. With a different definition (GDP for instance), perhaps it will; I mean we likely won't even have nation-states so the term will be rendered moot.

We don't have a nebulous computer-game asset called "production" we can arbitrarily allocate toward either useful or wasteful ends.

No, we have literal production, whereby human labor is combined with capital and natural resources to output goods and services.

There's a lot of resources simply wasted on worthless ends.

I mean yeah, but waste generally results from production. Even just setting fire to a tar pit is a form of production. But reducing waste and increasing efficiency doesn't necessitate reducing production, and as far as I can see, there's no historical precedent for that.

It's like unburned fossil fuels: depending on the outcomes we want, the best choice may be leaving that coal underground.

That's choosing not to exploit a potential natural resource, which means not producing with that particular resource. We do that all the time. We don't dismantle our houses for parts. We don't create nuclear energy from all the atoms in the universe. The choice not to exploit a particular natural resource doesn't have any impact on the overall productive capacity of a population unless that resource is somehow a bottleneck like grain, which there's no reason to believe fossil fuels would be.