How true is this?

Indeed, only a handful of people in the entire world can code review cryptographic software like OpenSSL. It is very likely easy to hide theoretical (theory in the mathematics-sense) backdoors in open source since the cryptography community is super secretive (only privileged people have access to academic literature on the subject!).

>>15
I know what a backdoor in a crypto primitive is and I understood what OP's quote was trying to say, the problem is that nobody use the expression "theoretical backdoor" in the field because it's wrong and it shows that you don't know what a theory is in the first place.