[It may not be Jean's responsiblity, but he can't help getting involved anyway. He can say it's for the sake of normal people who wouldn't be able to revive if they died when Reiner shifted into a titan. That was at least half of the truth. The other half was too complicated for himself to actual start telling anyone else about it.]

He did. I was afraid to leave him alone so I stayed with him until he calmed down. That's why he told me why he was so shaken up.

[Most likely since Jean could very well relate to the story. Or just because Jean was trustworthy. Who knows.]

It's not that simple for him to just know on a moment's notice. His last memory is that. He's in a world full of enemies. He hasn't gotten anyone to talk to about it until now, since there's no way he's going to want to tell Gabi. So he's had to keep the memory as his burden alone. Especially since he's convinced everyone else has forgotten it even happened.