I added a brief note to the text to clarify.
The model is simplified, which is stated clearly in the text. How much wind and solar Germany can export in future is very hard to predict, and is far outside the scope of this piece. In many respects it is down to what renewables its neighbours develop and how the market is working.
Timing of refuelling etc. is part of the reason France curtails nuclear less than the graph. But at the same time at 80% penetration French nuclear reactors still have to load follow, which is effectively the same thing as curtailment. From memory French nukes have average load factors of 78%, which would be closer to 90% if they didn't load follow.
As a rule of thumb curtailment problems will begin around the same penetration point as the load factor, so 40% for the Falklands sounds about right.