At the very end of the book—yes, past the epilogue and the acknowledgments—is a section titled "Top-Secret. Swipe Data Recovered. Group A. Subject: Teresa."
So yes, this is a memory block from Teresa—who, by the way, got her memory back in The Death Cure.
So the memory starts: Teresa is in a cold room in WICKED's headquarters.
She'd been there for two years now.
One of the people she sees sometimes is named Randall, but he only shows up occasionally.
(That's Randall Spilker btw—one of the people Mark read about on the workpad.)
A woman enters Teresa's room, and Teresa expresses her frustration that she never gets to see Chancellor Michael.
She also asks the woman for her name: it's Ladena.
Ahh, Ladena was the woman whom Randall would exchange emails with on the workpad.
Anyway, Ladena lets Teresa know that she's taking Teresa to where they're keeping the boy. It's time for them to meet.
The boy (Thomas) and Teresa will be given special responsibilities.
They reach Thomas's room, where McVoy is waiting for them.
Hmm, Katie McVoy was the woman who thought of the whole virus idea.
Teresa enters the room, and Thomas introduces himself. Teresa and him seem to get along.
Katie McVoy tells them how they'll be the leaders over the other children, and that they'll be the keys to finding the cure. Also, they can communicate telepathically and they have to help construct the Maze.
A few hours later, Teresa and Thomas have some alone time, where they chit-chat for a while.
Teresa asks Thomas what his story was—why he was brought to WICKED.
Well, first Thomas's father caught the virus.
Then his mother caught it. When they came to test Thomas for the virus they found out he was immune. He was pretty pumped about being immune, but now he feels guilty for being excited.
Then Thomas asks Teresa what her story was. Teresa says she doesn't like talking about it.
Thomas says he talked about his past, so Teresa should tell him.
Then Teresa drops the bombshell: her name was Deedee before she came to WICKED. And she tells him the rest from there.