A single example of twins who are similar proves nothing. I've personally met identical twins who couldn't be more unlike each other - which is no more evidence than the single case (case study, anecdote, they mean the same thing) you cite.

The author works as a professor of statistics, and his work is specifically in the field of uncovering the latent variables which underly human achievement. His arguments seem cogent and clear to me. Just because he comes to different conclusions from you does not mean he "has an axe to grind", which is a purely ad hominem accusation. Where do you think he makes a mistake?

He has admitted that part of his skepticism is due to the fact that he doesn't like the concept of intelligence in a general sense. He is far more critical of claims of heredity than he is of environment. Even half the level of scrutiny he applies to twin studies would invalidate nearly the entirety of sociological research on account of "not controlling for genetics". That is why I say he has an axe to grind.

As for his credentials, he's an associate prof with little to no background in biology, psychology or the study of the brain.

Steven Pinker, on the other hand is a full professor at Harvard (formerly MIT), and is the world's premier on the subject he's writing on.