Edit: Changed title to be more accurate.
Also here is the summary from Wikipedia on what Post-scarcity means:
Post-scarcity is a theoretical economic situation in which most goods can be produced in great abundance with minimal human labor needed, so that they become available to all very cheaply or even freely. Post-scarcity does not mean that scarcity has been eliminated for all goods and services but that all people can easily have their basic survival needs met along with some significant proportion of their desires for goods and services. Writers on the topic often emphasize that some commodities will remain scarce in a post-scarcity society.
Because the comparison is between systems. In other systems some people often end up with nothing, as in not even enough food to survive. That happens less under capitalism, hence there’s a more even distribution of resources.
Kinda like two pieces of paper are both thin, but one of those pieces can be thicker than the other one, despite still being considered thin.
People end up with nothing under Capitalism as well, and there is less disparity in some non-Capitalist systems.