I wouldn't discount the idea African slaves had something to do with these crops. In the Americas we have the arrival of different African crops from melons, gourds, sorghum, okra, species of rice, cowpeas, plantains, and maybe more. Out of the millions of people trafficked it's not hard to believe some carried seeds. As mentioned, Europeans contributed to diffusion and cultivation.
It's evident some of these crops have ranges across Asia too, and likely even arrived in Africa from Asia at some date millennia ago. I think the same happened with cannabis. In Phylos, sub-Saharan cannabis clearly has a link with SE Asian varieties. Some of this is probably Indian influence, but with the arrival of the ancestors of the Malagasy off the coast of Africa, plus crops like bananas in Africa, maybe this is another current. Maybe something complex happened in the Americas too, such as Asia > Africa > America + Asia and Europe > America.
I've also asked this before, but where does the American word "doja" for smoked cannabis come from? Depending on the answer, this word could be linguistically significant because it sounds a lot like "dagga", a native South African word for cannabis. I've searched for etymology on dagga and supposedly it comes from Khoikhoi 'dagab' or something similar. Found nothing for doja except a 20-year record of use in hip hop lyrics.