Rubinius wants to help you make Ruby better

I dare say that she knows her own intent better than you do. It is also unsurprising if her formulation is imperfect.

In a practical world what the LSP means is that you should be able to replace an object of a given type with another object of any subtype and it should still work. If it doesn't, then that wasn't really a subtype. As you correctly pointed out, the challenge in dynamic languages with exception handling is that it is possible to write code that depends on a particular method not existing. Similar code in C++ would give a compilation error. Therefore no subtype can quite be perfect in some languages.

Still even though perfection is impossible, it is good to be aware of the principle, and adhere to it as closely as is practical. Meaning make your changes be one that will break the smallest amount of code that your code change is likely to encounter at any point in the future.