Did you skip over the word 'assure'? The implication is that other projects aren't performing that particular assurance.

Problems such as a plague of racist jokes aren't omnipresent, but they show up often enough in the world that a little signpost at the front door about expectations can help with first impressions and understanding the community.

If this were a signpost along the lines of "employees must wash hands", then there would be no problem. Of course employees should wash their hands.

The problem is the weaponization against random people for obscure reasons. The blog poster here didn't make a presentation full of racist jokes, it's not even clear what they did.

Lawful evil anti-social people exist in the world, we shouldn't let them bully people just because they're waving a rainbow flag while doing so.

Maybe I'm misunderstanding, but is the idea that--apart from a CoC--people might venture into a community thinking racist jokes are okay?

A lot of people will venture in thinking that maybe they're okay here, and they'll find out later.

The implication being that the coc gives clear guidance on what to do if it does happen

Fun straw man though