Ruby is a dynamic language, `version_string` can be anything. The author uses `to_s` to coerce it into a string. There are problems with that: if I pass in an array it'll coerce into `"[1,2,3]".split(".").map(&:to_i)`, which makes no sense.
Most times it's better to just accept the dynamic nature of the language rather than do this kind of runtime type checking. You'd have to do this `.is_a?` dance for every type to have it be reliable.
Even if you implement an "interface" (duck typing) with `respond_to?(:to_app_version)` you still can't be sure that the return type of this `:to_app_version` is actually a string you can call `split()` on.
