I think he's operating off the assumption that it will branch from the Gladiator class like Paladin does. Even then, however, there is not evidence to support that it would be a tank job. Both Summoner (DD) and Scholar (Healer) come from the Arcanist (DD) class.

There is no reason that Paladin (Tank) and Dark Knight (DD) can't come from the Gladiator (Tank) class. If anything, this would be a great DPS with both damage and survival skills from Gladiator.