I don't believe in the concept of "fun in general" in the sense you imply here. I've played very successful games that have evidently achieved popular appeal but that nonetheless turn me off because they're the absolute opposite of Dwarf Fortress: carefully manufactured designs of intentional experiences that you are guided through at every step. Some people really like that, some will want to be thrown in the wilderness with no clue, some want something in-between. Different strokes for different folks.
So are you wrong in saying generally that it's "despite its terrible onboarding, not because of it", or am I coping by lying? Mind you, Dwarf Fortress is preceded by a whole genre of more traditional Rogue-like games that made losing hundreds or thousands of times for a lack of knowledge of the games' systems before you figured them out a fundamental part of their appeal, so I'm not exactly alone in this.
I think you want this game to be something it's not. It's not a casual game for casual gamers. It's a game whose mantra is literally "losing is fun". It's esoteric menus and attention to detail are the result of decades of work by a couple of guys that just wanted to make the game they and their fans wanted to make.
It's imperfections are part of it's charm and if that turns you off from the game then it's not for you and that's OK.
You're being way too dismissive of people that think the onboarding is reasonable. They can disagree with that without being blinded and thinking the game is flawless.