I don't know what you didn't understand, but that actually just proves my point. Posts are bound to an IP (& email), and IPs (& email) are considered as private data, i.e. data which can be used to "directly or indirectly" identify you ("personal identifiers", "online identifiers"), thus, posts are personal data, because per definition this is data which relates to an "identifiable person who can be directly or indirectly identified in particular by reference to an identifier".
The thing here now comes in another wording - companies can decide if they want to provide a minimalistic data export (containing user provided data only such as email) or a full data export containing everything, including user-generated content.
Anyways, just looking at any major company handling data access and data portability would have shown you how it's meant, but he, I guess, Google, Facebook, Tinder and all those tech-companies with heavy user-generated content haven't understood GDPR aswell, right? They are probably just providing your generated content to you because it's fun I assume. Tons of traffic and data storage, sharing their precious collected stuff with you. Ever thought about why they do that? Not like they have anything to gain from it.
Actually, the fun part is, that these data exports existed even pre GDPR on many services. Some weren't free, some not in digital form, but whatever. You could have migrated your data including user-generated content prior to GDPR, but with GDPR you have a right to demand for it - in a structured, machine readable way. However, it's the companies decision if they want to fulfill your demand.
I don't know how it was regulated before GDPR, or if it was regulated at all, but for a fact, GDPR does include that.