Shannon's entropy theory disagrees. No matter how good the best AI is, its ability to hide a backdoor is limited by the information density within the programming language, and the complexity of possible backdoors will plateau, allowing much lesser AIs to discover them. Consider that binary software is just a more dense form of code, too. A much less dense form, Javascript, makes finding backdoors easy because they all have to use obfuscation via base64 or something to avoid static analysis, which itself immediately draws suspicion.