What Nynja is saying is that if you allow credit card companies to dictate what games you make, it follows you will allow them or someone like them to dictate what TV you make, or what movies you make, or any content.
It's like saying "Why have laws that enforce freedom of speech if it's not affecting you directly".
Because it affects everyone, everywhere.
They didn't indicate what games you can
make in this case though? They only pressured platform(Steam) and the platform agreed with their demand.
Game makers can still make r18 games with any content, and either sell it on their website, different website or use patch to add r18 content (people have been doing that for years)
I don't see it as hindering freedom of speech because platforms have as much freedom to make rules as game makers making their games.
It's like, if a forum decided to ban certain topic is it hindering freedom of speech? The forum get to make the rules, you can say things on different platform or on the street instead.