Those are additive, they don't affect the semantics of code that was written before they were added. That is what is meant by backwards compatibility here - you never need to change existing code because of new features.
before es6 anything added also did not break existing implementations. eg you could run es5 features in a es4 engine using polyfill and progressive enhancement/degression. maybe its time for ES to add meta and overloading - although with a huge cost of complexity !?