[relaxing]

> “That’s all they do” and “just” are interchangeable.

I disagree. Whereas “do” is a verb, “just”, as an adverb, implies an entirety of being, an essence. And a system is not solely its actions.

[globnomulous]

I think the person you're answering is correct. "They just do X" and "All they do is X" are logically interchangeable. They define the exact same set -- and convey the same dismissive tone.

[relaxing]

You used the word do in both cases.