It should be noted that the data in question here may be personal in the sense that they belong to the user but not in the sense of the GDPR.

Whether the GDPR apply to data stored on that service can only be judged on a case by case basis after having looked at the data in question.

In any case, I'm not sure that they prevent their customers from deleting data. It seems that that content is public unless deleted if a customer cancels their paid subscription.