Honestly, this is a terrible paper. Unequal exchange is not a good way of understanding the world as it is. The only real unequal exchange under Capitalism is the payment of wages for labor. (Yes, there is rent of various kinds and some other things maybe, but they are very small compared to surplus value extraction). To make it more concrete, US farmers selling corn to Mexico is not unequal exchange, even though it may take 1 hour of labor to make corn equivalent to 10 hours of labor of avocados that exchange for the same value. On the other hand if a US company employs workers in Mexico, it is exploiting them to make profit which filters back to the US without equivalent. There is of course a legitimate point to make that if the world economy were truly as open and integrated as the Neoliberals sometimes claim, these production differences should equalize. One of the big reasons production methods don't equalize is that wages are artificially suppressed in 'global south' countries. There are lots of other reasons though which would be interesting to investigate more, rather than clinging to unequal exchange theories.

edit: In fairness they do kind of acknowledge this. I think they underestimate the differences in physical production. Take India, a large part of the agriculture is not mechanized to this day, so of course it is 10-100x less productive.

It is important to note that, in cases where physical productivity differences do exist, this is often because it is more profitable for capital to use cheaper, more labour-intensive methods than to invest in modern equipment—especially in cases where state investment in technological development has been curtailed by structural adjustment programmes, or where patents prevent affordable access to necessary technologies—precisely because Southern wages are maintained at artificially low levels34,35. This arrangement benefits Northern consumers with cheaper goods and benefits Northern capital with an increased surplus. In such cases, the use of labour-intensive methods facilitates value transfer and should be understood as constituting unequal exchange. Under these conditions, the South is compelled to allocate more labour to production for international trade than would be required if technology was deployed more rationally and fairly, thus draining—and wasting—a crucial productive capacity that could otherwise be allocated toward producing goods and services necessary for local well-being and development (see Supplementary Discussion 2).