https://warcraft.wiki.gg/wiki/Drust
Some vrykul settlements were scattered prior to the Great Sundering. After the Sundering, various vrykul tribes set out for the seas, and one such population of seafaring vrykul settled on Kul Tiras.
All vrykul suffered from the curse of flesh, which made their offspring turn into regular humans. The thornspeakers came from the Drust
Know your lore next time before make an assumption like that.