There are many situations where a post being visible to anyone is harmful to someone else. We can rationally weigh the value of freedom of speech versus expected harm to that individual and come up on both ends, but we can't ignore this simple fact when discussing this issue.
The only example mentioned in the post is child pornography, but there are more others: revenge porn, doxxing, smear campaigns to name a few. If my ex wants to send an explicit video of me, and someone else wants to view it, I am still harmed by the fact that the platform makes this possible, even if my moderation filters don't show it to me. Similarly, if someone is spreading my address or saying I eat children, the fact that I can choose not to see this doesn't protect me from the consequences of others reading these messages.
Again, not saying that I believe it obvious that such messages must be removed. My only point is that the "solution" that Scott proposes for avoiding harassment is a partial solution at best, but more realistically, entirely useless. It basically only helps for very low level harassment, such as not wanting to see someone cuss.
Content A is filtered-by-default, demonetized or otherwise discouraged and distribution-suppressed. Content B is not. That's effectively how all propaganda works. Even a totalitarian state can't really prevent access to content. They make inconvenient to access, and unwise to produce. That's enough.
In fact, modern propagandists intentionally leave a "steam valve." China isn't that worried about shutting down VPNs or whatnot. Firewalled-by-default is enough and overdoing it can be counterproductive.
I think the solution is awful. It doesn't work at all for the most important cases. If someone is being abused, libeled & harassed by a belligerent ex... hiding revenge porn behind and "adult content" filter isn't good enough.
If a platform is filtering political content, the fact that it can, technically, be accessed by enabling "harmful content" does not make it less censorious.
YouTube is in fact a great example of this being a real problem - they infamously chose to reduce the visibility of non-mainstream news channels, drastically cutting their viewership while not removing a single video from their platform. They also often de-monetize videos for even mentioning certain words or subjects, greatly dis-incentivizing anyone from discussing them (e.g. rape can't be discussed on YouTube if you want to make any money from your video).