The entire flashback with Grisha being rescued by the Owl, Eren Kruger, is being retold by Eren as Armin writes it down and Mikasa listens in an adjacent cell. Eren is able to provide this information from many years ago thanks to his coordinate status.
Among the things he learns is that once given the powers of a Titan, a subject of Ymir will only live 13 years, something Mikasa dismisses out of hand, as she’s probably committed to making sure her beloved Eren lives to at least 100.
Kruger doesn’t have any comfort or solace to give to Grisha, because he was never given any himself. He’s only been able to survive as an Eldian spy within Marley by actually acting the way a Marleyan would; injecting his countrymen and casting them over the wall one after the other. By the same token, he deems Grisha so suited to save Eldia because he has already set that path into motion by leaving the gates of the ghetto with his sister.
Leave it to Titan to break up all that dourness with a couple moments of levity, such as when Eren seems to pose when he repeats what Eren Kruger called his Titan: the Attack Titan. This is funny on several levels, as Levi waves it off as latent chuunibyou on the teenage Eren’s part, while the older Hange is ignorant about such things. More than that, though, Eren finally gets to say the title line—a title that in English perhaps never should have had that confusing “on” in it…
There’s also the suggestion that for the duration of their imprisonment, Mikasa only ate the bare minimum to stay alive, and thus was literally wasting away without Eren by her side. But their sentences are commuted and they’re free to go, seeing as how they’re heroes of humanity and all.
“Free to go” is relative, however. They’re out of the stockade and back in uniform, but they are ordered to attend an audience with none other than Queen Historia, who feels a connection to Ymir’s letter similar to Eren’s with the books and photo they found in the Basement.
While the words of the letter seem to be not much more than the “love letter” they appear to be, it’s clear Ymir sent it in its form in order to deliver far more and different information to Tory; she’s just not sure what to do with it, or even whom to tell. She’s just glad to see Eren, Mikasa and Armin, and embarrassed when they all bow before her.
At the hearing, Hange delivers the report full of revelations provided by Eren and recorded by Armin, all about the reality that their kingdom within the walls being but a tiny sliver of the real world, and that the vast majority of that world is dedicated to their destruction.
As he listens to the testimony with everyone else, the new memories provided by Grisha continue to swirl in his head. He notes that the Titan into which Dina Fritz transformed was the same Titan who ate his mother and Hannes. When he met that Titan later and touched it, a similar surge of information suddenly flowed into him.
Now he knows why, and he almost blurts it out, but thankfully Hange can sense why he ends up holding his tongue, and chalks it up to his chuuni phase to the assembled bigwigs. What Eren now knows is that it’s possible for him to gain the vaunted powers of the Founding Titan—the main mission Kruger gave Grisha when he sent him to the walls—by touching Historia while she is a Titan.
He’s loath to bring this up because he doesn’t want to see Historia suffer any more than she already has. But what if, like Grisha and Kruger and so many other subjects of Ymir before them, he’ll have to sacrifice something important in order to gain that which will restore Eldia.
Or perhaps not; perhaps Eren is supposed to break that cycle. After all, another part of Kruger’s mission to Grisha was for him to fall in love with someone in the walls, raise a family, and love them. Now there are people Eren loves, and perhaps there are lines he won’t cross, even for the sake of saving the world.