Yes, exactly. Making something illegal is serious.
The result of making something like this illegal is:
1. Person is not allowed to talk OR
2. Person is allowed to talk but is now compelled to only espouse a message approved by the government.
Illegal speech in the US is speech which does harm or has a direct incitement to harm such as specifically calling for a violent action, shouting fire in a crowded theater, or lying about someone to harm their reputation and cause financial impact.
To make something like denialism illegal would require you to show that, by allowing someone to say it, they are causing direct and immediate harm. That's not the case here at all. Saying the holocaust didn't happen doesn't cause people to then go commit genocide. At worst it convinces people some horrific event didn't happen, but that horrific event is still horrific conceptually.
Denying vaccines work may convince people not to get them so maybe you'd argue direct harm there? But it's not clear to me how you can measure the harm since it's arguable that said unvaccinated person may get covid and be totally unphased. What about those people that got covid prior to the vaccine? How could you argue direct harm from them when they already have the antibodies sans the vaccine?