Are death threats a viewpoint, or an action? The law treats them as the later. They aren't protected by 1A. There may be a handful of extreme absolutists who want to protect death threats. I'm not one of them.
Again, is deliberately spreading misinformation (as opposed to genuinely believing something that happens to be wrong) the same as expressing a viewpoint? No. When deliberate misinformation is targeted at an individual, it's called defamation. Not protected. Theoretically, you could more broadly prosecute deliberate misinformation without discriminating against viewpoints. In practice, I'd be fearful of overreach.
Thing is, the flat earthers while obviously wrong aren't doing much if any harm. That society is basically in complete agreement that these people are nuts means society is no real danger. But it's entirely possible for society to be confident and also wrong. Fostering such a spirit of censorship doesn't have much of any benefit when society is correct, but is massively detrimental when it is incorrect.
And when society is split, you are never going to find an example as cut and dry as flat earthers, which of course will not stop anyone from claiming things are cut and dry. So let's suppress communist ideology because it is "discredited and might claim another hundred million souls if it's allowed to spread." Let's ban all pro-BLM viewpoints because its narrative falls apart when you control for confounds and the racial division is damaging. And let's ban all discussion of vaccine safety because those people are "anti-science and dangerous to society." And maybe they are, but God help you if you ever succeed in suppressing all anti-vax sentiment because the incentive for big pharma to make safe vaccines will drop off a cliff.
Or, we can tolerate each other and debate. Even if it means we need to put up with a few crazies and we aren't ever going to convince the zealots. If the "dangerous other" side is still claiming converts, then maybe instead of censorship and suppression we need to improve our messaging, or maybe we aren't as right as we think we are.