That's a myth until i see proof or read conformation about that.
Things like that have been stated again and again, but for now it's nothing more than a forum theory that went viral.
I have written evidence regarding Solo / Group NPC interdiction in places that should not have such a large population of NPCs in the first place. There's a Interdiction Mega-Thread somewhere that I listed the incidents in detail.
Basically, in a set of 16 zero population, unscoopable star systems, about 200ly from Sol, I had an encounter in 13 of them. Everything from an Eagle to an Anaconda. All of the ships that managed to interdict me had the same 'give me your cargo / now you die, dog' comms. I was driving an Adder, with no cargo, no fines, no bounties.
There is no logical reason for any of those ships to be there. None of those I could scan had fuel scoops or exploration equipment, but there were tooled up, ready for bear, in the middle of nowhere. If they were NPC Pirates, they'd have very slim pickings indeed!
So, yes, there are instances of high numbers of NPCs showing up in Solo / Group and it isn't a myth or theory.