This article would therefore define ANY war as genocide. (in whole or in part, killing members of the group), and it may even be a “peaceful situation”!

People always focus on the wrong part of the definition.

The important one is the intent.

Wars are waged for various reasons - you need "lebensraum", you need oil, you intervene on behalf of the UN, you counterattack after being attacked yourself, etc.

The goals in these cases are: expansion of borders, hoarding of wealth, arguably humanitarian intervention, or military defence.

If your goal is to eliminate a people, that's genocide.

And yes, that's also the reason why it's so difficult to actually define a military action as "genocide" - because it's often almost impossible to unequivocally determine what was the intent behind an attack.

And with that, let's look at your examples:

This would mean the Nazis were genocided by the Allies.

No, because the goal was the stopping of the genocide of Jews, and defeating an aggressor that terrorised Europe and North Africa for four years.

This would mean Japan’s treatment of Aum Shinrikyo was also a genocide

No, because the death sentences were carried out not because of their religious beliefs, but because they committed acts of terror.

This is why no one takes them seriously

No. No one takes them seriously because most people, like you, don't understand the definition.