I don't understand anyone calling their load a .380 +P since there are no SAMMI specs for it, but having said that most manufacturers are loading their .380 ammo at around 14,000-15,000 psi/cup (which BTW is target/cowboy pressure levels off the SAMMI spec) because of all the legacy blow back design pistols on the market, like the Bersa Thunder. SAMMI max spec for the .380 Auto is 21,000 psi/cup. Could it be that they are simply loading to the SAMMI spec and denoting that with the +P tag? I would ask them at what max pressure levels they are loading their +P ammo. If its equal to, or less than, SAMMI spec then its OK, over that and they are taking quite a chance since no one is really building overspec .380 Autos that can take advantage of the overspec ammo.
The truth is that the new locked breech design small .380 Auto pistols handle the higher pressures that are .380 Auto SAMMI spec much better than the larger blow back design pistols. I would be much more confident shooting max SAMMI spec ammo in a TCP, LCP or P3-AT than say a PPK or Thunder.