For example, T-Mobile is a platform. They aren't responsible for anything you say when on the phone, using their network.
CNN is a publisher. They are responsible for anything that gets posted on their website, and can get sued accordingly.
Social media companies want to choose what is posted on their website, but also not be held responsible for anything that is posted on their website. They want the perks of being a publisher, and the perks of being a platform.
Obviously there are arguments made on both sides. But that is the general disagreement, if I understand correctly.
A website that contains both first party and third party content has section 230 protection on the third party content, but none on the first party content. There is no legal basis for the idea of "publisher" as different from "platform". If a website makes a modification to a specific piece of content, they may lose section 230 protections over that particular piece of content, but they retain it on all other 3rd party content.
The FCC cannot change the requirements to gain "platform protection" because there are none, because section 230 applies to content (like a tweet), not entities (like "Twitter").