You run out of other people's money. You can squeeze labor to starvation working in a salt mine. However, if most all people lose all their money, capitalism is done, and currently runaway capitalism is doing everything it can to increase that disparity.
This person was referencing the obtuse and infuriatingly repeated quote from Margaret Thatcher (rot in piss) "The problem with socialism is that you eventually run out of other people's money".
I don't think I need to point out exactly why this quote is stupid.