Well, let me try to explain it this way.
Hypothetically you are using the 12/2 at the m4/3 which equals to the fov of a 24mm lens. Your subject is 3m in front of you.
On full frame, the 12/2 will still be a 12/2 and to get the same 24mm fov like the m4/3, you'd need to move closer to 1.5m to get it.
And we all agree that the closer you are the shallower the dof.
And I think its a rough estimation of the exact dof of the 12/2 = 24/4 (dof and fov speaking only). I read there was an article about it somewhere. But the crop applies to the dof also, not only the fov. The light gathering ability stays the same.
However, if you use the 12/2 on fullframe but crop it digitally later to get 24mm fov, and you stand at 3m from your subject, then the dof stays the same. But you get a wider fov that you can crop around.
Imho.