Humans are not born as tabula rasa. Our behaviors are partially genetically determined. But we are born with a very high level of plasticity allowing us to configure to our cultural norms and the local langauge, for example. Lewontin and Gould were afraid of the social implications of this knowledge, that it would be used as an excuse by racists to terminate certain genetic lineages, and they thought they were doing the right thing by denying the truth, especially given that the holocaust was just 30 years prior to this debate. But denying an inconvenient truth doesn't make it go away.