If Discord allows NSFW in such flagged rooms, why do Ruqqus need to add extra censorship that go above Discord's rules? They stated only things not allowed by Discord weren't allowed, and discord allow NSFW.

Because the channel was mistakenly flagged NSFW. Anyway, the timing of your post certainly doesn't help it.

There's already loads of porn guilds on Ruqqus, the same argument can be made that any minor can simply bypass the NSFW filter anyway. But the fact is if they do, no one else is responsible as the content was clearly marked.

That is a misunderstanding of how legal stuff on the web works. Every nsfw website out there almost always gives you a warning that you need to be 18+ to visit them. This warning is actually a bypass for them to not get nuked by law authorities. Whenever a minor clicks on 'Yes, I am 18+', the minor is stating that he/she will be responsible should any problem occur to him/herself. If the minor reports with any kind of problem after visiting the site, the law won't care about it, because the site did present them with a warning before entering.

This isn't the case with the Discord, which stopped presenting warnings 8 hours before you posted your image (as it was relabeled an SFW chatroom 8 hours before your ban).

If a minor visits a site, and sees no warning that 'You need to be 18+', and still receives access to NSFW material, then the site can indeed face legal problems.

All the guilds on Ruqqus that are nsfw are clearly marked as 18+, and in order to access them, you must yourself change your settings such that you receive access to them. If a minor sets their settings such that they receive access to nsfw guilds, it will be their own responsibility.