Because the only thing we have to go by is crossover scaling, which we can't use, because it wouldn't make sense, it's like using Dissidia 012 in a battle for Tifa and saying she can beat so-and-so opponent because she beat Ultimecia, it just doesn't work.
We also don't know how truly powerful he is.
In any case, The Living Tribunal > IG, so he wins this battle without too much of a struggle regardless.
He's nigh-ominipotent and has the ability to completely erase both Galeem and Dharkon from existence, even if we counted every single one of their feats at face value.