OK, the recoil spring that pushes the bolt forward is a heavier spring than that which keeps the barrel in place. I haven't looked at it closely enough but the basic idea is that the barrel "floats" and it moves that small amount (.030" which is less than 1mm) before the spring is overcome by the recoil forces acting against the case. The whole thing is remiscent of the Russian's Abakan design where the barrel floats on a carriage.
If you look at those super-slow motion gunshot pictures you'll see the bullet is already downrange before any of the recoil forces can act on the gun and shooter. Inertia, baby!
Also, FN coats the P90 cases with a clearcoat that has slippery qualities to aid in extraction. Quite possible this is not needed on a semi-only gun but it is a voodoo subject right now since no one can figure out what it is to replicate it. Personally I plan to dry lube all of my reloads but it is worth mentioning as this may assist in the extraction on this blowback system.
If I were to make one I would definitely try to imitate the delay system somehow, or use a gas-operated system to avoid the blowback issues altogether. Perhaps using .22 Hornet or some other similar type of round.
Or just build one of these