Okay
Yeah, there isn't much to take from. I mean, she might have been, but it will be only speculations I guess. Your speculations are as good as mine though(or better!)
I would guess the order Tyrande talks about here is the Highborne, and how the following Nightborne(group) "exiled" themselves from the rest, but that could also mean what you think it means, an order of mages etc. I mean, the Nightborne is a race of mages and warriors.
Just wanted to know if you actually got some lore, its nice to read/get info. From the start of Legion I have searched, checked those tablets in actual Suramar ingame with text on etc(crazy). I would think they made their own order of leaders/titles etc after they actually went under the dome, but that is just my guess!