It's not necessarily pollution. Soils in some areas have high naturally occuring levels of cadmium, lead, arsenic, etc. You can search and find articles about high cadmium levels in corn, sunflowers, wheat, barley etc. It depends on where it's grown.
Might be true in some instances but these cocoa farms are well established.
cadmium in the garden (extension.oregonstate.edu)
cadmium in soils (sciencedirect.com)
From another article: "However, Cd bioavailability is complex, for example, rice from the southern part of China contains more Cd than rice from the northern part of China. Possible reasons are the more acidic soils, an overuse of nitrogen fertilizers, pollution through irrigation and atmospheric deposition, and the cultivation of rice with a higher affinity for Cd accumulation in southern China (Chen et al., 2018; Yang et al., 2016)."
Not just China.