You're right. However when there are copyright violations on Reddit, they claim it's a platform and they're not responsible for the content on it - ultimately the users are responsible. And then the courts agree and decide that ultimately the violation rests with the user and not with the platform.

However, as they increase the moderation of their platform, they move closer and closer to being responsible for the content. At some point of restriction, they're completely responsible for the content on their platform.

They're welcome to operate the site however they want, but they shouldn't reap the advantages of a public platform while advancing their private interests.

seems to me that there is no "completely" unless they literally screen and evaluate every single post before accepting them. until that point's it's all matters of degree.