The answer is in the article. MooTools and Prototype both replaced Array methods that were later standardized, but they still work, because their provided copie

The answer is in the article. MooTools and Prototype both replaced Array methods that were later standardized, but they still work, because their provided copies overwrite the standard versions. MooTools went further by copying “all methods from Array” onto another MooTools object, “Elements.” THAT broke, because the standard Array.flatten would be non-enumerable, and so it wouldn’t get copied to Elements, and so Elements.flatten would fail.