To me it looks like weight. The aero changes are small, Ferrari was stable like a rock on setups so far...all the change with this better tire management and simply higher pace comes in my point of view from the weight reduction.
So I also think this will be responded, Ferrari will reduce weight with the updates, Merc sounds to me like having a big weight reduction in their coming update that they bring as a package to save money (what Ham meant with "should have done")...
So it will now be a run for updates bringing down the weight to the limit. The real pace of the cars will be visible once they are on the limit.