the solution is to remove the bits of those apps that are harmful to children (and adults): the algorithmic data feed, the infinite scroll, the engagement tactics, the advertising

So, for instance, pornography and gambling should be 100% illegal? Or at the least, all social media sites should censor any discussions that aren't child-appropriate?

but how can you remove adult-only features for children only without knowing the age?

They're saying we should remove the features in general because they're anti-features harmful to everyone, and focusing on children distracts from that fact.

This conclusion is up for debate, but that's what they mean.

We should ban oil, drive EVs, everyone write Rust only, and invest in index funds. We should, but it's not going to happen.

don't do harmful things is pretty easy and can apply to adults too!