I'm not saying that it's right to lump people together like that and BPDs in particular don't take well talking shit about them.

However, some people with mental issues in some situations really pull off lies and deception which blow most people's minds. I've been in situations where people in charge wouldn't believe some things happening right under their nose because those things were simply too bold and, well, insane. And because people are easily manipulated into forming good opinions about others and apparently don't like learning they have been wrong.