It's not that it wasn't significant at that time. It's just that in the long-term, it looks small compared to making the value of the labor of the entire population 100x more valuable.
The correlation between historical slavery and industrialization/modern wealth is negative.
Not really difficult to disentangle.
And theres Canada.
Whereas claiming they can't be disentangled is obviously disinterested.
Slavery was the rule, not the exception in the world at the time. The African kingdoms mostly practiced it, the Arabs were infamous for it, Mexico ended slavery in the 1830s after American industrialization was well underway, Russia didn't truly end serfdom until around 1900 as I recall, and then you have India's caste system, so only China is left without slavery in some form? But were they? And why didn't all of these countries industrialize?