MC NEVER implies that Gustaf is the hero of men. Ever. in any way.
Actually it does..very much so..
it talks about how the hero of men got the piccori blade and then later in the game, king gustoff shows up to talk to Link and you get the vibe that he is talking to you as a hero. It makes you think that the reason he was ever a king is because he saved the world. Go replay MC and you will definitely see why people get confused by it. Doesn't the opening scene use Gustoff's name in the BS
If you do decide to replay to see if it gives off that vibe, don't go into it thinking you are already right. There is no point in going back to check it if you're not even gonna have an open mind.
It DOES give the vibe off that Gustoff is the hero of men. If it didn't, then no one would ever debate it. Regardless of it being Gustoff or not, there is plenty of other stuff that puts MC far before AoL.