Basically this.
But that’s why there are some Thalassian elves who refused to join the Horde.
Alleria windrunner And Vereesa windrunner being one example as the Windrunner Family was almost extinct due to Trolls and Orcs. Her brother’s head was put outside of Quel’thelas on a stick. He was killed by a orc. Her mother was killed by an Invading Amani (who at the time were part of the horde).
Then you have the Allerian elves who left during the second war to fight the orcs, they are still Alliance.
You have the Highvale elves who are still loyal to the Alliance.
The Dalaran elves are mostly neutral, but lean more towards the Alliance.
Theramore elves who may or may not be alive.
And finally the Silver covenant.
Now we have more Thalassians joining through Void elves.