Like saying you don't need to be a structural engineer to demolish a skyscraper. You need to be pretty well versed in computer science to be able to hack a system to any reasonable extent. If you just go ham at it and just press random buttons nothing is going to happen / you're going to be detected.

Hackers write programmes all of the time.

To create a linux distribution under your own control you don't even need to engineer anything new. You need to create deployment structures but the tools for repository hosting, package managers, build services, automation and pki are already there. It's more about curating software and sometimes auditing or changing code. Someone who's capable of reverse engineering Blu-ray player firmware should be able to learn this as well.

Edit: 'A new distro' is also a spectrum. It's a big reach from creating a debian distro with some version differences to something like openwrt or asahi linux