You're right - all of those things are possible. What I'm arguing is that given you have a reasonably good test suite and are aware of what patterns to are dangerous, you can refactor fairly confidently with the tools given

For example, it's possible to fetch a class's entire interface before and after a refactor and validate it is the same. It's possible to dynamically wrap every method you're refactoring track them and type check them. And if you extend that idea, now you can output this data to files and perform static analysis. Sure, it all relies on you having some safe execution context to get this information but Ruby probably has the best testing tools of any language and many projects have great test coverage

For library owners, I agree with you - there's no hope. But for application developers maintaining their monoliths with nothing depending on them there's a lot you can do to ensure safe refactoring