Strange, I read that as well. They seem to be every where for me. I think the real delicacies are the Protonic Inversal Throwing Axes instead of the normal Proton Throwing Axes. I have yet to find a PITA, the PTA's are everywhere.
Yes, these are rare, the normal version is scattered everywhere and is easily encountered, but the Protonic Inversal Axe not. I've only found two in the game and at one time a Lobotomite was carrying another one.
The throwing version is also rare, I have no more than 9 of these.