The banner was indeed Highborne - perhaps not exclusively Highborne, but predominantly Highborne - in its origins and heritage. I've done some serious contemplation, some research, and there are several likely explanations for this change:
1.
The Shen'dralar were instrumental to the Teldrassil evacuation (the last portal was maintained by several mage trainers) - the surviving night elves seek to honor their efforts on their people's behalf - even if most of the Shen'dralar are probably dead.
2.
Farondis and Kur'talos also use this banner in the Black Rook Hold and Crumbled Palace (in Azsuna and Val'sharah), so it is not necessarily exclusive to Suramar City or Elisande's people; perhaps there are other conclusions that could be drawn, or...perhaps not.
https://i.pinimg.com/originals/a4/82...deca229163.jpg
3.
Tyrande performed the Night Warrior ritual at Bashal'Aran (one of the oldest Highborne settlements which the night elves have reclaimed), which is now their new base of operations in Darkshore; perhaps there is some connection. Maiev and Jarod are commanders there - perhaps they use the banner out of sentiment and nostalgia to their ancestors; especially after their experiences in Val'sharah encountering the ghosts of their former friends.
4.
The night elves wish to avenge more than Teldrassil; they wish to avenge their entire race - as their ancestors would have done in the distant past; perhaps Teldrassil's banner is a reminder of their losses, and they wish to largely abandon it out of respect for their dead (like the blood elves changing their quel'dorei banner), not unlike the humans using Lordaeron's symbol despite the kingdom belonging today largely to the Forsaken.
But there are other things that make little sense and logic. Why does the Nightsong reference Zin-Azshari - named after widely despised Queen Azshara, Tyrande's most hated enemy - why not sing a song to Teldrassil and Darnassus, or perhaps Ashenvale and Kalimdor the continent as a whole, in line with their nature-reverence theme? Why do the kaldorei seek to gain a foothold in Azshara - when it was but a remnant of the Highborne capital of the ancient empire - a reminder of the Sundering and their controversial past?
Crown of the heavens
Glory of Azshara
Children of noble birth
Crown of the heavens
Elune be with you
Elune be with you
Crown of the heavens
Elune
Glory of Azshara
Glory of Azshara
Be with you.
I'm exaggerating a bit here, but this scenario seems possible:
I don't really think the scenario would necessarily occur, but it is not impossible. Certainly, Tyrande is sometimes an easily angered person - that was why she was so obstinate in reforming society after the Sundering; Varian told her to have a little patience, remember? And she was very harsh to Anduin - she moved for immediate action during the War of the Thorns - and afterwards, before the Battle of Darkshore. One wonders what a conversation between Tyrande and Thalyssra today would end - would they try to make amends, or would Tyrande lose control like before, and simply kill Thalyssra, Valtrois and Oculeth upon sight?
Mistake me not,
it is a beautiful and ornate banner - incredibly ancient and powerful simultaneously, invoking an unimaginably rich and glorious past - but BOTH night elf players and nightborne players can relate to it simultaneously, does the shal'dorei not also make use of the banner even today? The only problem is - and some nightborne players were a bit upset or at least confused by this particularly - is that it could be construed as appropriation, like Tyrande suddenly marching into Suramar after Teldrassil was destroyed and declaring that everything there belongs to the night elves simply because she was born there 10,000-15,000 years ago.
For most Horde players at the very least, it is hard to ignore, the night elves not using this banner for ten thousand years, and then suddenly using it once again without clear explanation to either side. Would not most blood elves be understandably upset, if Alleria began suddenly using the Icon of Blood as her racial symbol despite siding with the Alliance, just because most void elves were "formerly" from Silvermoon, or if most high elves began using their night elf ancestral symbols in their settlements?
P.S. Does this banner really appear on Mount Hyjal? I thought they did not update Nordrassil?