I don’t think this would be extended to the Darkspear given Vol’jin and the Darkspear Tribe’s efforts against both Garrosh and the Zandalari, and their very vocal distaste for both.
Given both the Darkspear and the Zandalari’s opinions about each other, their recent quarrels, and their differences in culture, I’d say the Nightborne are closer to Zandalari culturally than they are to the Darkspear, number of fingers in their hand aside; and thus, here goes my own take of this headcanon!
The Zandalari and their culture is in a strange spot amongst the Horde. While not particularly openly reviled by most of its people, and certainly helpful when it comes to military affairs, the Zandalari feel a little ‘isolated’. The fact that they allow trolls of tribes not sworn to the Horde visit Dazar’alor for religious purposes and trading, rightfully earns the chagrin of plenty a race in the Horde, with the Terrace of the Tribes’ Darkspear Embassy being armed enough to almost be considered a small fort.
To an outsider, unknowing of their history and their culture, one could guess that the Nightborne, the Blood Elves, and the Darkspear would get quite along with them, and find common ground with them; and yet, the rift between them is not only greater than the would-be-bridge, but it is mostly devised by the Zandalari themselves.
They have fashioned themselves to be an empire of great might that isn’t just the direct continuation of, but the very same empire that quarrelled with both of these race in both ancient and recent history, and they take a pride in it that is too verbal for the comfort of anybody but them, whenever a foolish, particularly patriotic recruit begins to reminisce about ‘Ye Good Olde Zandalar’.
As for the Zandalari themselves? It is not any more shocking than it is for everyone else. Everyone seems to be quite proud of fighting, if not wiping out, their ancestors, allies of their ancestors, and deluded and misguided close relatives of them in recent wars from Zul’s deceit.
Furthermore, their culture of castes of high birth and great usage of the Arcane would make one of them think they’d see eye to eye with a blood elf, or their great chronicles and love for history making them see eye to eye with the pandaren, or perhaps, even finding some common ground in spirituality with the Darkspear, thinking themselves to be the arbiters of all things Loa.
And yet, there they stand, at a crossroad they all have paved themselves. They are not quite like the elves, not quite like the pandaren, and not quite like fellow trolls either, and they surely aren’t any closer to the rest of the Horde races - and perhaps, somehow, in a twisted sense of harmony, all four of them are fine with not being each other; especially if the Zandalari are to be the missing link between them.
As for the Shal’dorei? The Nightborne are just very competitive and pretty pissed off at the Zandalari’s claim that they are somehow the pinnacle of knowledge and power in the Arcane and other matters magical inside the Horde ranks. Theirs is a relation less about history and their ancestors, and far more about the rivalry of two magical nations duking it out to be at the podium of Arcane magic in the Horde, while some would argue that they are merely fighting for second place, with the Sin’dorei taking the crown.
It is not rare to see an argument on this matter extend from the realm of speech and into the fires of sword and magic in plenty a Horde War Magi academy, or to see books flying like homing missiles in magi towers, citadels, and even the Pathfinder’s Den.
