The definition of the word capital is private ownership of the means of production. The means of production do exist under every system, that is true. Private ownership, on the other hand, does not exist under every system. The basis of private property is control of something that you intend to extract profit from, and do not intend to use yourself. Your car and your computer are not private property. The same factory may be private property one day, and not be the next.
You could say anything has the potential to be capital, but that's a pretty useless statement.