I cannot speak to the mathematical errors, but I recommend you take most of what Motl says with a grain of salt. He is well known in the physics and mathematics community for a variety of interesting statements, including my personal favorite: "Discrete math is just a disorganized mess of random statements." http://www.scottaaronson.com/blog/?p=1720 Which is interesting, because I have yet to see a construction of continuous mathematics that didn't start with something discrete.

Motl is an infamous pain, and makes enemies at the drop of the hat, and is extremely scornful and overly negative -- but he's also intelligent and knows mathematical physics.

I would disregard his unsupported opinions out of hand without thinking twice, but not his supported arguments, whether right or wrong.

I would pay to see Motl and Doron Zeilberger in a cage match about discrete math: http://www.math.rutgers.edu/~zeilberg/

I have the book in front of me right now (I originally read it around 2007). It's very sparse on actual math: I couldn't find anything that could be considered a mathematical treatment of String Theory. It's more about the academic culture surrounding the theory.

Disclaimer: I'm not a physicist.