I was thinking the same thing. Mortensen goes on and on about plastic lighting.
And I have a paper by Heinrich Kuhn, the inventor of the Imagon and who died in 1944, I think, where he talks about "The plasticity of the pictures (the Imagon creates)....."
Given the goal of a lens like the Imagon was to create "dreamy", idealized portraits, I suspect plasticity in his context implied smoothness and softness of texture, not sculptural and defined.