Your mistake is thinking the purpose of these laws is to stop "nazis." It's not. The purpose of these laws is to provide some legal backing to silence critics of the government. They will never make a law that says you can't criticize the Government. But they don't need to, all they need to do is make a law that says supporting terrorism is illegal, then it's easy to squint and say that agreeing with a terrorist organization is the same as supporting terrorism.

For example, if the green party of Australia wants to stop coal mining or whatever and ELF blows up a coal mining truck, suddenly the green party of Australia is breaking the law by existing so they have to spend all their effort defending themselves against the law, rather then attempting to ban coal mining.

That scenario is the purpose of this law, but with governmental support of Israel. Every time a public figure criticizes Israel they have bend over backwards and spend the majority of their time claiming how much they love the Jewish people and definitely aren't Nazis, and now if they don't sufficiently prove their non-naziness, they are suddenly breaking the law and now there is another avenue for people who want to silence critics to pursue. It's not a coincidence that this law was passed on Dec 8th.

That's what the slippery slope is, the silencing of dissent, not the specific verbiage of the law.

For example, ask yourself why https://en.wikipedia.org/wiki/Kurdistan_Workers%27_Party would be be banned under this law?