How do you define an OS? You can write an EFI "OS" that prints "Hello, world" in approximately 5 lines.

It's a bit more than that.

> We'll implement basic context switching, paging, user mode, a command-line shell, a disk device driver, and file read/write operations in C.

see https://1000os.seiya.me/en/

that's not an OS, that's an EFI application