It's a value that was set by the other french mod, Adrien, back in 2015. He could probably give you more explanations regarding the why's of this value, but as far as I'm concerned he deserves it. I personally have him rated 83 in my OF, but it isn't much of a difference really.
The thing is that Rabiot is a player that's technically extremely safe when it comes to ball retention. His physique has nothing to do with how good he is with keeping possession, he's just extremely composed and technically good enough to do it and that's why he's rated 84.
I'd like to point out that this season isn't the best to judge PSG's midfielders' technical abilities. The whole creativity thing has been switched to Neymar's side. Midfielders are supposed to help build and keep possession, this season they were all like "Let's get the ball from Silva/Marquinhos and let's give it to Neymar/Mbappé". The transitional phases between attack and defense in this team really got worse since the signing of Neymar. Mbappé is more of a team player, so I don't blame him for that, but Neymar really tends to "vampirize" the ball a bit too much.
There's also the fact the team is far less possession-based under Emery. They tend to have superior possession because they're technically dominant in their league and they're used to play a possession game, but Emery tried to bring more verticality in the team, resulting is much, much less time on the ball for the midfielders like Rabiot. This is also one of the reasons why players like him didn't show their whole technical ability this season.
Same thing could be said for Verratti, Draxler or even Lass Diarra who's also known, to a lesser extent, for being comfortable on the ball.
Everyone knew it was impossible. Then came along a fool who didn't know it, and he did it.