When people chose not to name a variable selection specifically because they mix it up with the call to Model#selection I absolutely think it’s based on a lack of knowledge about the syntax.
Regarding readability I think model and selection reads infinitely better than mod or ss. Mod could mean module, modulus, modification or any other word starting with those 3 letters. The only way to know what it means is to scroll to its definition and see where the value came from, or learn that it usually means a certain thing in this developer community. On the other hand, someone who has never written, or read, a SketchUp extension before could easily tell that a variable called selection refers to the selection, and one called model refers to the model.