Twitter allows porn. How do they do it?

Twitter is probably big enough that more Apple users would complain if Apple enforced such a hardline policy, and has a pre-existing relationship with apple (If I recall correctly, Twitter and Facebook were the first two share with opitons on iOS), so Apple is more likely to forward on complaints than nuke them? Twitter also requires more personal data (e.g. phone numbers for new accounts), so that may discourage users from posting illegal content in the first place.

Yes. But to remove only child porn and not all porn requires you to have some way of determining what is child porn. So you need to sort it into "child porn, remove" and "porn of consenting adults, allow".

Or you do what Tumblr did, and just ban all porn.

the person you replied to was being unnecessarily semantic, but in computer science "sort" has a specific meaning which is only the ordering of a set. So 'sorting' cp implies making it easier to find specific cp.

The more accurate word might be "categorize" or "filter".

It was obvious what was meant by "sort" when reading the full text, but I think the counter-point was more of a tongue-in-cheek retort regarding the above than an actual complaint.

"tongue-in-cheek retort", "pointless pedantry and pointscoring", it's all a matter of perspective I guess.

If you read beyond those three words, I think it's very clear that they mean removing.

It is called divide and conquer. Sorting is the hard part, filtering/deleting/alerting is the easy part.