Eren nods slowly. He still doesn't entirely get how two worlds could be in one place at the same time... but he also doesn't get how you can have two rooms occupying the same place in Liminal Space, even though that clearly happens in the Workshop. He assumes overlapping worlds would work much the same as those overlapping rooms. Which he has no idea how or why that works, but he knows that it does, and that's good enough for him.

"...So that... that thing still had a hold of him the whole time!?" Eren shudders. "The rest of us didn't know his Mark would bring him back! You could have at least told us that much! When Malik said that Crawling bastard ate his ghost, his soul, I..." Eren's voice lowers, quiet enough now he can barely be heard over the roar of the engine. "I was scared he was just gone. Even when he came back on the Network, I was so scared that he might be left behind when the Jaunt ended."

So it really was Armin's own strength and resolve that let him find his way back to them like that, instead of something the Arcana did. For just a moment Eren smiles faintly, proud of his friend for continuing to fight through despite what had been done to his body and mind - and for winning. But then he frowns again as Cairbre continues. "No," he says, speaking more loudly and clearly again. "I couldn't. Not really. You just said the world of the living and the world of the dead are separate. We ended up in different worlds and on different Networks. He reached through to us on so we could help him get his head straight, then again to make the plan, but he couldn't do it a third time. For the rest of the Jaunt we couldn't reach each other at all! I had to just sit there knowing how much he was hurting, and I couldn't do a damn thing to help him!"