Think what he is saying is that the code itself has backdoors that allow bitcoin to be stolen. Example: https://arstechnica.com/information-technology/2018/11/hacker-backdoors-widely-used-open-source-software-to-steal-bitcoin/
You could argue that close sourced is more risky. But open source also means that scammers have full access to the code and can readily identify vulnerabilities. This is one reason why Ledger does not open source all parts of its architecture.