They might have same features from the perspective of checklist. But path they developed and obtained those features is different, the syntax is different and there are probably also some subtle (if not big) differences in semantics. The similarities in feature list isn't result of 2 language "implementations" getting closer to shared specification, but at least partially looking at what features other programming languages have and then sooner or later developing their own version of those language features. I haven't read the ES4 specification or development discussions for ES6 so I can't tell whether differences are caused by ES6 classes being developed from scratch ignoring how they where described in ES4 or whether classes in actionscript were only very loosely based on the way ES4 described them. Either way the development of two languages at some point forked and further developed somewhat independently.

From what I understand graph is something like this: ES2 -> ES3 ES3 -> ES5 ES3 -> ES4 ES3 -> AS1 ES4 -> AS2 AS1 -> AS2 AS2 -> AS3 ES4 -> AS3 ES5 -> ES6