Eren would probably have never believed it if he hadn't seen the photo there in his father's journal. Even then, it had been hard to stomach, to understand. He'd known, somewhere in his messed up hidden memories, that it had to be true. Touching Historia's hand and unlocking most of those memories for good solidified it.

"It should be," Eren said, "but it's not. My dad escaped from Marley after Zeke sold him and his wife out. His wife turned into a titan for good, but…someone on the inside gave my dad the Attack Titan."

He doesn't really need to go on from there. They know Eren has the Founding Titan; that's why they want him. The Attack Titan, well…he's not certain how long it hid outside of Marley's control. Since before Grisha had it, must have been, but that part is fuzzy for him. It doesn't matter.

"Obviously Marley wouldn't want any of that getting out. They only told you what you needed to know. And inside the walls…we were lied to for a hundred years. You know that already, right? You knew we didn't know shit. We didn't have technology like you did in Marley. We stayed in the dark and we would have died like that, like animals in cages. Instead, I set us free."

That's an oversimplification of the hell that Eren will bring to the world. He knows that. He knows Bertholdt might hear the truth of the future from Reiner. Why overload him with it here and now?