No idea.
At some point Google started promoting the possibility to package an Android app as a Chrome OS app, using a tool called ARC Welder.
So as far as AquaMail is concerned, it's running on Android, and expects everything it normally "gets" in that environment.
I don't know if full contacts integration is a part of this "Android running on Chrome OS" mechanism.
To AquaMail, the code works with "Android" contacts, using respective Contacts related functions defined by Android. Just like it does for everything else.