> one of the main purposes of which is to store and give access to the public to copyright protected works or other protected subject-matter uploaded by its users

That sounds like it includes comments by users (since their comments would be automatically copyrighted)

> the content is uploaded with the authorisation of all rightholders concerned

The owner of a comment board has no way of knowing if their users will include (in their comments) content to which they don't have the authorisation to do so.

It seems pretty clear to me that it applies to comment boards.

"(4b) ‘online content sharing service provider’ means a provider of an information society service one of the main purposes of which is to store and give access to the public to copyright protected works or other protected subject-matter uploaded by its users, which the service optimises. Services acting in a non-commercial purpose capacity such as online encyclopaedia, and providers of online services where the content is uploaded with the authorisation of all rightholders concerned, such as educational or scientific repositories, should not be considered online content sharing service providers within the meaning of this Directive. Providers of cloud services for individual use which do not provide direct access to the public, open source software developing platforms, and online market places whose main activity is online retail of physical goods, should not be considered online content sharing service providers within the meaning of this Directive;"