I reckon this is more about the FIA not being able to say quite how Ferrari have been managing get extra Fuel/Oil to burn without it registering on the metering system.
They know Ferrari have been up to something but any proof of wrongdoing has been elusive, they can't penalise them without being able to explain just what the wrongdoing is - hence the need to offer a confidential truce.
Basically they've agreed to take no action so long as Ferrari came clean about how they manage to do it. Now they understand how it's done they can ensure no one else is doing it, or can do it.
Really there's a reason on both sides to keep it quiet - Ferrari don't want to admit to any transgression and the FIA wants to prevent anyone else doing anything similar.
Obviously Ferrari have managed to keep this technical fix/code/innovation secret up to now - FIA wants to make sure it stays that way.
The only way I can see to defuse the row, now brewing with all non Ferrari power teams, is for the FIA to admit that for once Ferrari boffins out foxed them, embarrassing as this may be.