That begs the question: how do you differentiate between an opinion and a harmful falsehood?
This isn't something MR can solve. Heck FB and Twitter and Reddit and pretty much the entire internet is struggling with this question. I don't think you can differentiate between the two, and thus the difference is irrelevant.
We all agree that falsely yelling "FIRE!" in a movie theater is dangerous, not protected speech, should be censored, and has not led to a slippery slope of many bad things. It's a harmful falsehood.
Does saying "In my opinion, this theater is on FIRE" make it any better? Classifying it as an opinion doesn't make it any less dangerous.
I feel the mask thing is the same. Saying "masks don't work" and "in my opinion, masks don't work" are equally bad. The former is an falsehood, the latter is an opinion; they're both equally harmful.
Some (let's be honest, it's mostly the conservative right) have weaponized this idea that "opinions" are automatically protected speech no matter what. But it's bull. The content/context matters, whether it's in the form of an opinion or not.