The failed idea behind making drugs illegal is that they'd be harder to access and less people would do them. There's evidence this didn't work.
I think the idea here is the same - if the big platforms ban certain types of egregious and harmful misinformation (that gets recommended to people susceptible to conspiracies) - these companies are hoping less people overall will watch this content, and probably more importantly, if anything bad comes of this, they won't have their brands tarnished because of it.
Will it work? Who knows. They're companies. Why can't they do what they please as long as it's within the law? Does the law require YouTube to keep up all content? YouTube has a Terms of Service and has censored lots of content for a long time. Why is there such a big distinction between beheadings, child porn, anti-vax, hate speech, and election misinformation? These companies already have a history of censoring "political" content for foreign countries.
What is new here?