It is an achievement (achievement #158). It will pop up at the end of negotiations, not during the endings. (So if you were an alpha backer, you might have gotten it already)
Okie dokie, I'm completely sure I've gotten all legendary endings before and seen the on-screen achievement pop up, but it's not showing in the achievements page. I re-did the ending and the achievement didn't pop-up on screen, and it's still not in the achievements page.
Similarly, I got the "You went up against Gisette. You lost" achievement during gameplay, but it's not in my achievements page. Could this be related to the clean slate version? I did wipe everything once, but the other achievements have been showing normally as I get them.