Okay then it wouldn't be protected by GPDR, but instead Copyright Law (since you are always the copyright holder of your own stuff) which can't be diminished by companies terms of usage.

The GDPR states that data is classified as “personal data” an individual can be identified directly or indirectly, using online identifiers such as their name, an identification number, IP addresses, or their location data.

And if these online identifiers give information specific to the physical, physiological, genetic, mental, economic, cultural, or social identity of that natural person.

In some circumstances, even information related to a person’s job, hair color, or political opinions could be classed as personal data. Usually, this comes down to the context in which the data was collected and whether a data subject could be directly or indirectly identifiable.

https://www.gdpreu.org/the-regulation/key-concepts/personal-data/

So pretty much any significant discourse you have on a platform would be covered under GDPR as an EU citizen. They also have a list of examples on this page too.

This is indeed the correct interpretation of the GDRP rules. (Handling and processing of) Personal data, NOT content, are protected/restricted.