I believe the real reason was that FDB was not open sourced.

Jepsen tests cannot be used to prove the system is safe, but to prove it isn't.

It looks like very often he looks into source code in order to figure out how the system operates, that way he can find weaknesses and write test to his testing framework to demonstrate the issue.

I wouldn't trust any company that uses Jepsen to show that their product is safe.