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!).

It's not easily to hide backdoors, and what the fuck is a "theoretical backdoor in the math sense"? who wrote that?
Also the "cryptography community" is everything but "super secretive": there are tens of mailing lists, forums, chat rooms, blogs, websites, and events that are open for anyone. There are also thousands of books and papers that any kid can download.

Your quote OP, is the most stupid piece of text I have ever read.