By allowing extremist content unhindered on your site, you are giving them a platform, you are helping them reach more people. You are in fact helping them boost their signal, rather than adhering to some ideal that ordinary people can somehow talk down extremists.
You cannot reason with a true believer. You cannot use facts to debate someone for whom the entire concept of facts is fluid and nebulous, something that only matters when they can twist it to their own ends. If you let extremists run free on your site, they will take over the narrative and infest every community they can.
It is impossible to debate someone who doesn't care about truth, lies or facts. They will spin a web of arguments that is impossible to counter, because they are not bound by what is real. The people who try to counter-argue have to stick to facts, so they get bogged down in sealioning, gish galloping, bad-faith arguments and outright trolling.
The end result is that by engaging the extremists, you allow them to repeat their arguments ad nauseum while you try to counter-argue with facts, but those take time to look up and state coherently, whereas the extremist can just make up lies as they go. It doesn't matter for them, what matters is that they appear to have all the arguments and that they appear to have the upper hand, because all that matters is projecting power.
The best way to get rid of extremists online is to ban them from your community.