Watson was vastly simplifying genomics, as is this letter. To grasp the details requires knowledge of class, culture, nutrition, and genomics.
it reflects a pre-1990s dogmatic belief in extrapolationist macroevolution.
It treats high heritability + polygenic additive variance as if that somehow proves the Modern Synthesis was right about everything
It never once mentions developmental constraint, niche construction, active rGE as a macroevolutionary ratchet, cultural-genetic co-evolution, or any of the other mechanisms that the post-2000 literature now recognizes as necessary for understanding why human g exploded in the last million years
YMMV