I’ll answer that.
The Nightborne may have been physically changed to shalssians by the Nightwell. But they are still a night elven group and are a Kaldorei culture. They are just a sub group of that. A magically induced variation that isn’t a separate non Kaldorei elven group
just like void elves are still a Thalassian group in a Thalassian culture despite being magically altered.
Same with Lightforged Draenei.
They may not be the original races strictly anymore b/c of their magical changes, but they are very much part of that racial group set even if a separate faction of it.
Suramar and the Nightborne are 100% night elven culture and civilization. Just like Lightforged Draenei are 100% Draenei culture group.
The Nightborne focus on a part of the Kaldorei that is very much Kaldorei and is exactly that side and city of the Kaldorei. You should know that. Just because they are called shal’dorei and their ears look different should somehow make this hard to see. Maybe it’s hard because you don’t see that side of the night elves shown in game. But it’s very clear in the books.
Maybe if they had varied the void elves body a bit you would realise it’s the same thing done there also.
I also think if blizzard had shown that side of the night elves in the shen’dralar like they did for Suramar it would be much easier for players like you to see what I am talking about.
No one should need to point this out to you. Nightborne, like the other allied races are not new whole races void of the group they are based on. They are sub races, variations of existing races entirely based on those main races that explore a different story for that group one that changed them enough to be a playable sun race but not totally removed. And the Nightborne are entirely based on the night elves in their era during the War of the Ancients and their culture which just wasn’t shown fully in game till Suramar. And we all know the shen’dralar in Dire Mail and Darnassus are night elves in that culture.
It is still Kaldorei whether the current Darnassians don’t live that way or only a small group does. Whether it was lived that way 10 years ago or 100,009 - it is a Kaldorei thing.
if you can’t see that the Nightborne are a Kaldorei group, or can’t see the void elves are a Thalassian group or the Lightforged a Draenei group, the Zandalari a troll group or the Highmountain a TAUREN group. Then that’s on you. Just because their name is different and each of them had a magically altering experience - it’s so clear.
Maybe you never read the night elf books or don’t know much about them, but Suramar is a night elven city and culture. And so are the Nightborne who are the rightful owners of Suramar.