They are driving around a limitation that F1 cars have always had, which is the wide variation in aero performance which is mainly dependant upon the front ride height, the rear height, vehicle yaw angle and steer angle. By reducing the peak longitudinal acceleration on entry (but getting higher overall acceleration (lat+long)), they are making it easier to drive the car to closer to the limit throughout the corner, thereby maximising overall acceleration and hence reducing laptime.
I can see why they use the term flatter platform, but it is not a term that I have ever used and is not quite correct. What they are trying to do is to minimise changes in front and rear ride height through the corner and the car at its most efficient will have a pitch angle, i.e the car is not flat and if it were flatter, it would be slower.