Yes, it's true... the .380 hasn't been subjected to the SAAMI pressure-setting limits that determine what a "+P" load IS. This is likely because it's never really been considered a primary duty cartridge by any Leo departments... which means that if it's not a .38 Special, 9mm, .40 S&W, .45 ACP, or .357 Magnum (and a few select others), no such designation exists!
This doesn't mean ANYTHING, in practical terms. Cor-Bon's .380 loads didn't come with the "+P" rating on the side of the box originally, but now they do... a voluntary admission on their part that these weren't to be confused with Wallyworld ammo.
Most of the hotter European loads would likely fall into the +P category.
However, there's another thing to consider. The P-3AT is a locked-breech pistol... and the .380 was designed to operate a blowback pistol. In effect, this means the P-3AT could (and does) handle any .380 ammo, from mild target rounds to handloads in the 9mm level! These loads would reduce your Walther PPK or Bersa to scrap metal.