You have to keep in mind "no platform" refers to the message of hatred, bigotry, and violence that our typical fascist has to share.
We're not saying "I arbitrarily disagree, and thus it is justified to silence this person!" We're saying that giving a fascist space to recruit other fascists to their cause leads to them attacking people or otherwise acting on their stated desires to violently oppress people.
Think of your Spanish Civil War. No one offered to engage the fascists in debate, because fascists aren't interested in space to discuss their beliefs. They are interested in space to develop an air of legitimacy and look strong.
Why is it so hard for you to understand that nazis and fascists are more than theoretically bad? When you sign up to be one, you make the conscious decision to advocate for national socialism, you are advocating for the systemic dehumanization and murder of people, there's no way around that. This isn't like targeting someone who may be violent under the right circumstances. This is combating people for whom violence is a prerequisite for their very existence. The world is more complex than your banal hypotheticals ever are.
if a Nazi or Nazi group harms us or threatens to harm us (which they will always do), then we fight back, violently if necessary. But if all they want to do is walk around talking about their bullshit, then no, we do not physically harm them. I would say the same thing about any group. It's not about their ideology, it's about not legitimizing the use of force against political demonstrations based solely on how much you hate them.