Earlier quoted context omitted.
That looks like ActionScript 2, so the javascript convention was actually well in place by then. I can't speak confidently about the late 90s, but I wouldn't be surprised if it predated Actionscript 1 as well. But doesn't it all trace back to C conventions anyways?
Not at all. AS1 was plain ES3. movieclip._x or movieclip._alpha (as in position and alpha blending) were accessed by properties with underscore to denote that you were using accessors (getters/setters). Internally properties like Object.prototype.__proto__ had two underscores to denote they were private. In AS2 you had: class Foo extends Bar implements Xyz { private foo public function set x (value : Number) : void {…
Programmers using underscore to denote identifiers for internal use was well-established and in use long before ActionScript even appeared. To claim that it's a "direct descendant" of ActionScript is arbitrary, anachronistic, and odd.