The simple answer is that renaming a variable (or function parameter) is a semantic change - even if it doesn’t, say, change the compiled code, it affects how a human would read and understand the code. Plus, in many languages, function parameter names are indeed semantically meaningful and can be accessed via features like reflection or keyword arguments.

Yep, that's correct. Difftastic is only a syntactic analysis, although it often makes the semantic intent clearer.