IIRC Jensen's original argument was based on very high estimates for IQ heritability (>.8). When within-group heritability is so high, a simple statistical argument makes it very likely that large between-group differences contain at least a genetic component. The only alternative would be that some unknown environmental factor would depress all blacks equally (a varying effect would reduce within-group heritability), which is not very plausible.

Now that estimates of IQ heritability have been revised down to .5, the argument loses much of its power.