You're not talking about them behind their backs though, no one knows who you or they are. It's anonymous. It doesn't affect people's opinions about the person. Talking about someone behind their back would be talking to people that know you or to people who know the person. That will affect people's perception of a person, that can be unethical. There is nothing wrong or unethical about posting there anonynously.

I agree that the sub has fundamental flaws and that you can't see the nuance of a situation, that people aren't very objective and they're biased towards themselves a lot of the time but that doesn't mean it can't give you some perspective provided you're not just posting for validation. It's not a sub for legal or relationship advice, they even state as much. Everything has its downsides, some people will lie to themselves and believe they're right even when they're not and others will gain some humility by realising they were in the wrong.

Everything on that sub is an opinion and personal perspective. It's not a source of undeniable truth, its a guideline and what the posters choose to do with that information is up to them. There is nothing unethical about that.