Or, they themselves could attack their own software if there's a benefit. They won't reveal you the manner which they discriminate coins, so why wouldn't they? It can go completely unnoticed.
Well, that's another possibility because they can sometimes attack their own software and may act as hackers to find the flaws that hackers could find with hacking tools. Sometimes they also might run bug bounty programs and pay hackers to find those bugs in their software.
This is so true. In fact, it's been noticed that many times developers who write closed-source software do follow the "security-through-obscurity" principle. People try obfuscation techniques believing that provides security, in stuff like web apps and steganography. Believing that closed-source is more secure than open-source, besides debunked, is a sign of utter arrogance; believing that no one can contribute to the security of your project as much as you've already done is excessive self-esteem. Windows is tangible evidence that this is bad practice.
Everything that includes cryptography should be developed transparently.
I agree with you and most of the closed-source software are packed with obfuscation techniques by keeping security in mind while sometimes they use those techniques to prevent open-source software developers from copying of the source-code. Those closed-source people aren't generous at all and they don't want open-source developers to view the code and make something similar and allow everyone to use it and contribute to it freely. Such software could contain many vulnerabilities and when a hacker finds those vulnerabilities then he/she may do anything with the systems of the users that are running those software on their systems. I strongly agree with you that the software which has cryptography should always be developed with complete transparency so that any developer could had improvements into it.