I thought Gab was always trying to be a direct competitor to the big tech communication applications, which were considered platforms for everybody; you don't want porn in something like that.

There are quite a few artists who use Twitter for their content, many of whom post both SFW and NSFW art. They came from Tumblr after their porn ban or Pixiv and now primarily or exclusively use Twitter.

Those artists who post even occasional NSFW or borderline NSFW artwork cannot move to Gab if they want to retain that ability without having to worry. Users who want to view their artwork or interact will have to continue using Twitter for that purpose.

Something trying to be a competitor to 4chan is a different story.

4chan is completely unsuited to the kind of artists who currently use Twitter. They want a platform for both posting their art and interacting with their fans (and allowing their fans to interact with one another).

That isn't to say that Gab should be that platform. The amount of users who might move over to Gab if they didn't have the ban could be minuscule, after all.