Because they’re correct, you’re just being too literal in your understanding of the statement.

Will people occasionally work for free? Sure. For friends, family, friends of friends, hell even strangers sometimes, volunteering is a thing, sure.

Will they work for free reliably and consistently enough that they can be built upon by other people?

Obviously not, they sometimes don’t even do it for money.

Society as it is today, with its insane population count and highly specialised workflows, requires an insane amount of logistics that absolutely can’t bear “random cunt #354 decided not to work this month so the boat is without a captain” levels of random disruption without heavy consequences; this is incidentally also why strikes are extremely effective.

No society that evolved beyond subsistence did so without some obligation to work, whether through monetary incentives or straight up serfdom/slavery; and if subsistence is all you want, I’m sure you can go live in like, some tribal commune on a Pacific island somewhere.