If this was true, the only hopeful sign was that they did take him—for if they planned to lay waste to Earth, why bother?  They could simply leave him to his fate on the beach, and open fire with their orbital cannons or whatever weapons they had.  They probably weren’t planning on holding him for ransom—any survey of the planet would tell them that they held all the trumps—so they were still pinning their hopes on using him as a translator, and apparently they valued him enough to want to keep him, rather than wait for someone more qualified to come along.