You're sort of misreading info
The embark screen shows what civs are in range (which change with location, of course), while the civ screen within the embarked fortress shows the civs that fortress has encountered. As caravans (or sieges) from different civs come to your fortress they will be added to the screen. I've never seen a black tower there, despite them attacking, so I don't think they count as civs. Both goblins and kobolds show up there after they've been encountered on your embark, though.
I've never seen any dwarven civ except my own, which I assume is due to only my own civ sending caravans. If you'd somehow ended up at war with another dwarven civ (if that's even possible), they ought to show up on the screen once they come aknocking.
Also note that in the latest version siegers actually travel from their site to you, which means they'll attack eligible targets in between before getting to you, and they have to actually be large enough to send a siege (a site with 10 goblins cannot reasonably attack with 50). This means sieges are rarer than before, so if you want a lot of them, you should make sure to embark close to the enemies.
One thing that took me a fair while to understand is that the selection of dwarven civ matters. Some might be dying, while others are in prime health (dying -> risk/chance for premature monarch election), and each civ also has its own relations to each of the other non dwarven civs (e.g. being at war with one goblin civ, but not the others, one being at war with an elf civ, but not the others, etc.). It seems your mother civ caravans are able to reach you regardless of distance (but caravans are not guaranteed to show up every time regardless).