It's not irrelevant at all. The fact that capitalism and markets tend to be found in the presence of one another, and that markets, under certain conditions, produce good outcomes, doesn't mean that capitalism is therefore good. If we're discussing ways of improving the human condition, it's actually an absolutely critical distinction. I don't even dispute the conclusion that private ownership of capital is actually a powerful means to economic development, but the key point is that this is conditional and not a starting premise - when those conditions are breached, then private ownership must be curtailed. This is not a controversial statement: as an example, we accept that private ownership doesn't successfully provide for the optimal amount of education.
As for private capital and economic growth not going hand in hand,
the World Bank is a good place to start. Some of these are a bit misleading (Liberia and South Sudan are both growing now because they have plenty of slack after decades of civil war), but the correlation between the degree of private ownership of capital and economic development is pretty poor regardless. The historical example also militates against it: most of the Asian countries that saw the fastest development in the post-imperialist era were ones which saw strong state control of key industries.