@Snarf Zagyg is 100% sure that double attacks is canonical. I OTOH always played as you suggest, that it was a straight single extra attack per round, and I insist this is not a 'backport' of the 2e rule, though it is literally impossible to prove that unequivocally at this point. I have some character sheets (a lot actually) but they are not really filled with rules references and such...
Suffice it to say that I agree that
@Snarf Zagyg has a perfectly valid and probably most commonly used interpretation, and that Roger Moore may have agreed (I'm not sure that his article really resolves this, but I haven't read it in like 20 years). I think it would be worthless to debate it further, AD&D is a game with very murky rules, many of which have been interpreted in multiple ways over the years. Who is to say that one interpretation is 'right' or another one 'wrong'? Gary himself could at best tell us his intent, but he's not here, so...