Of course money is measurable per se, but capital is not. The whole point is that the benefit of the owner is not linearly related to any measurable quantity attached to labour.
Capital interacting with itself does produce more capital.
And, from your explanation, I think that it is more a rhetorical word than a real point of the theory. Generically, the main problem I see in this focus is that it seems to imply that the whole production under socialism must be, at best, of the same order that under capitalism. For instance, is very difficult to show that destruction of capital is actually a legitimate source of income for capitalists.