I don't think that's true. Even Haas has improved the load on its car and did better in qualifying than the old Ferrari in all the high-speed corners (so more load ), but it has the same suspension. It is too simplistic to attribute the bouncing to a weakness of the suspension, in this specific case of the third element, the only real mechanical organ that can be considered suspect. Without the inerter, no spring-damping unit can stop vibrations above a certain amplitude any more, because it is not possible. You would have to go and change the parameters of fast compression and rebound considerably. This would alleviate the porpoising problem but would completely change the behaviour of the car on kerbs and bumps, leaving it unpredictable. This is why these values are roughly similar for every team in relation to the masses and weight distribution of each car. This is also why each team started to create holes in the diffuser to prevent porpoising, if they could have just designed a different third element we wouldn't have seen so many teams forced to give up aerodynamic load in order not to bounce around.
So, going back to Ferrari's problem:
I also thought at first it was only caused in rolling conditions by the change in height between the sides, but this is not the case, the rebound also starts in a straight line. I think there are other possibilities:
1) the floor is very sensitive to surface irregularities, so once the car has a bump and a quick bounce it stalls and then starts porpoising.
2) the floor has to operate at a lower ride height, so it has a very small working window and with the same load the car is lower than before and bounces
3) the stall starts when the skid touches the road and changes the flow conditions in the plank area.
Simply they have to understand better what happens with certain flow speed with the real condition, I suspect hardly they not saw that in wind tunnel because of this kind of variables.