Undermine isn't underwater; Kezan is still visible on the Maelstrom map of your world map within the game. Also, the city was under the volcano, so in all likelihood, it was destroyed and/or ruined to a certain degree, i.e. the perfect setup for an enterprising goblin to seize control as a Trade Prince. You're thinking (well, the poster that you quoted) of Zandalar, and even then, since it would make for an excellent expansion site, I doubt that they'll have the area underwater.