[godelski]

  > B would be inconclusive against what you'd hope to be some kind of background

Correct. And this is why scientists use null hypothesis testing. You disprove things in science, not prove them. I think that's why you're confused. In the first situation you disproved that it comes from the background

[vel0city]

I don't think I'm the one confused, as I'm not saying things like:

> On the other hand, if you found (say) 101 particles/unit in a sample of B, you'd conclude B doesn't do that

I'm fully aware of null hypothesis testing.

[godelski]

I interpreted that sentence as more casual language. Maybe I'm wrong to assume that. But I didn't write it either.

Either way, the difference didn't matter to answer your question. Them getting that part wrong doesn't make the other part wrong nor hard to understand.

  > I'm fully aware of null hypothesis testing.

Great! And now I know you know too