Whatever it was it had to be said in person with no email trail, text trail...no trace. That tells you all you need to know.
Also making sure that there were no bugs in the walls, no telescopic microphones. If I were the FBI investigators I would wonder if the AG was conspiring against my investigation. What could be so important that it required a top level face to face meeting?
I have to believe that it was an ambush by Clinton to make a direct threat to Lynch. Or maybe an offer of a supreme court position or other reward for Lynch if she finesses the investigation. This was orchestrated by Bill not Lynch.