A document like that was included in the original investigation, with 9 different chess engines. It didn't compare source code though, but on which features of the board each engine would consider when evaluating a position. For example one engine might give an extra boost to a position where your knight is in a central location and capable of affecting a larger part of the board, another might give a boost to situations with interlocking pawn defenses, etc.

According to the evidence, two engines with different ancestry would have a feature overlap of about 30-35%, up to 45% in the most extreme cases. With the exception of Fruit and Rybka, with 75% overlap.