He's not wrong. Certainly when it comes to the full scope of being a programmer, Linux and BSD are your only real options. You can't easily add a new filesystem or screw with the kernel on macOS/windows without having to buy hundreds of dollars worth of their books and attend conferences.
There isn't really any good detailed up to date documentation online about either. The docs for linux/bsd aren't perfect either but at least there you can always fall back on the source. That makes a massive difference.
For example, one of the reasons I switched from Java to Go back in the day is that I could actually read the source code of the APIs I was using. So I could fully understand the standard library whereas in Java it was all obfuscated bytecode. I'm not sure if the situation has changed with OpenJDK but my point stands, an open core system is far superior platform for learning.
There isn't really any good detailed up to date documentation online about either. The docs for linux/bsd aren't perfect either but at least there you can always fall back on the source. That makes a massive difference.
For example, one of the reasons I switched from Java to Go back in the day is that I could actually read the source code of the APIs I was using. So I could fully understand the standard library whereas in Java it was all obfuscated bytecode. I'm not sure if the situation has changed with OpenJDK but my point stands, an open core system is far superior platform for learning.