Mathematica is not a domain specific language, otherwise many of the examples in Rosetta Code wouldn't be completed…
See this[0] for more info
[0] http://reference.wolfram.com/legacy/v5/Tour/MathematicaAsAPr...
See this[0] for more info
[0] http://reference.wolfram.com/legacy/v5/Tour/MathematicaAsAPr...
There is no reason to think this looking at the website.
Since he uses median ratios, then a single spectacular ratio will not skew the results.
The part of the test set that people have actually provided Mathematica solutions for. There is every reason to believe that people are more likely to provide Mathematica solutions for problems that Mathematica is likely to be good at.