the mundo invested all this gold and all his itemslots in what you mentioned while for example a vayne has maximum attackspeed, 100% critchance and so much lifesteal that even if she doesnt condemn and doesnt tumble, mundo wouldnt be able to hurt her. also, his damage output is so low, if vayne buys 2 defensive items she is allready super tanky while being able to kill him in about 3-4 seconds.
also, its funny you say that "you disagree" on the defensive item scaling. its not an opinion here, they simply dont scale as good because they give diminishing returns. this means, buying more than one resistance item of any kind reduces the effectiveness of it. the only "exception" here could possibly be malphite because he gets the super tons of bonus armor, so if you'd go malphite against a full-ad team and you buy 12 armor items, well sure that could be fun.
still, there are resistance-shredding items but no real damage-reducing items except frozen heart and randuins to a very small extent.
in a game-mode like this, range and damage far outweight any tankyness or higher base stats of melee champions. this means champs like ap-kogmaw, victor, azir, vayne, tristana etc. they would be unbeatable. hypercarries like ryze, poppy etc, they would be insane.