I swear I remember seeing Machamp's Mega form as one of of the offical Mega forms with the artwork and it wasn't fan-made, but now, I don't see it anywhere? I'm so confused about this.
Nope, Mega Machamp is not confirmed as of yet. I can almost blindly vouch that what you saw was in fact a fan art, or at most a prototype design that accidentally leaked (highly unlikely). Many fan arts are pretty good and make a very convincing artwork (like this one), but the bottom line is that Machamp is not confirmed to have a Mega.
Hope I helped!