Aren't natural resources wealth? Isn't forced work wealth? "Stealing" seems like the best word to describe the situation without obfuscating the matter with grander narratives (the kind that might win you the "Nobel prize" for economy, incidentally).
You could also see it as a double condemnation of colonialism - not just immoral, but an economically useless endeavor.
Looking to the future, I'd prefer colonialism not be considered a lucrative strategy (though the thesis doesn't deny that colonialism was profitable for specific interest groups - just that those groups were a small part of the newly industrializing economies, and that the nation-level balance sheet gained little from their pillaging, compared to the costs of empire-maintenance).
You're implying that the colonial powers got rich by taking natural resources from Africa and forcing Africans to work without pay.
Since you don't offer any evidence for this scenario, I can't really refute it :)
But note that 90% of Africa didn't even get colonized before 1884. That was over a century into the Industrial Revolution era, during which Western Europe had roughly doubled it's population and tripled their GDP.