The argument is simply that the more things X people can do, the less things there are for a remaining Y people to do. When workers are more productive, total societal demands can be met with fewer workers.
Seems intuitively obvious. X factory produces N widgets. If it is can suddenly produce with half the workers, it will fire the others and get profit.
It has been argued that in these situations, demand will magically double or jobs will be magically found for these laid-off widget makers. Whatever their long pedigree, it seems to me these arguments only seem plausible because we've trained to believe them. A look at the present world seems to indicate that "what you'd expect without the propaganda" is happening - the laid-off widget makers of all sorts are thrown on the scrap heap with crocodile tears from the finance managers running things.
I understand that the real picture may somehow be different... For example, gains in healthcare may keep more people at work who else wouldn't be able to work. And an elaborate society also allows women to work, increasing the workforce. So I see this is not a black/white issue.