Earlier quoted context omitted.
Try asking your regular bank how their fraud detection works, or your insurance company, or the secret service. Basically if you work in fraud detection (or broadly any kind of crime detection) you keep your techniques secret because otherwise you're helping the criminals avoid you. Look at the source code for Reddit or Hacker News, notice how in the publicly released codebases they've both chosen specifically not to…
The security by obscurity approach usually give you only an ILLUSION of security
Then, you need to understand how open PayPal is to fraud. I can easily commit major crimes using PayPal, and security has nothing to do with it.
This is the challenge they face. If you think it's an easy problem to solve, ask yourself: why is there no one else out there doing this same thing? Why are all the other supposed opponents limited in such drastic ways that PayPal is not?
I'm not suggesting they are perfect, but your trite remark is meaningless and ignorant of the realities of the system.