This seems wrong or at best, not so clear.
Using the terminology from the text, it seems to me that a forum post is information relating to a natural person that can (potentially) be identified by reference to an online identifier.
For example, the posts in this thread are information, each one relates to its author, and each author can be (potentially) identified by an online identifier (i.e., his/her username). MDG already knows who many of us are, for example – that alone probably triggers the condition – it is very broadly worded.
Aside from MDG’s knowledge, a username is commonly anonymous, but that is not guaranteed (consider (self-)doxxing), and probably not a point that can be productively argued with a litigious end user.
Furthermore, what limited precedent there is on the matter does not inspire, see for example http://www.bbc.co.uk/news/technology-43752344.
The safe play would seem to be to delete all the posts of the user. I think some sites already do this, maybe Quora (looks like they compromised, so far anyway: https://www.quora.com/How-do-I-delete-all-my-questions-and-answers-on-Quora-at-once). Looks like StackExchange is trying to hang on to the content by claiming you licensed it, but I bet this posture will be mercilessly shredded by the GDPR (as it violates both the letter and spirit). [Can’t link to the SE page, my quota is exceeded] OTOH, StackExchange probably has a slightly bigger legal budget than any of us.
As someone recently tweeted, “it’s the GDPR’s world now, we just live in it.” Maybe re-assigning as ‘anonymous’ gets you there, but until we see precedent, there’s no way to be sure.
I am not a lawyer - this is not legal advice - find a professional.