Didn't they see him? Either they hoped to crowd Roland and get rid of him swiftly, nevermind anyone else at their back, or the former was the case. Tidus sees it happening before Roland's call, and he's working, thinking, trying to figure what will work best; he doesn't want to use an offensive spell at this distance when he can't properly gauge where Roland is and get him caught in it.

So he attempts a risk--a risk that may be better in Roland's favour, less so for the trolls. Tidus sprints to gain some better proximity to the towering figures, a wispy night-black gas gathering around the pipe in hand. Assessing the distance, the necessary force required and swing; and Tidus takes it, a propelling swing of the weapon flung across the space between them. There's no way it'll do damage when it manages to smack the troll across the back, but what it does it something else, a something visible from Roland's side: a dark smog shooting up across the troll's face, covering its vision, inflicting darkness.

Anyone abruptly shrouded in the dark are a likely wild force, but as much danger to their teammates as foe. And in the chaos, Tidus has stopped, taken out from his Arms the blitzball gifted to him by Saku. And with an easier shot, he sends it--with the same inflicting spell, to cast it potentially onto another troll.