Many people tell me that in order to write my own operating system I need to understand the inner-workings of the one I'll be writing it on. That's nonsense.

I mean I understand it for education purposes, such as studying the workings of a current OS to gain better knowledge of writing one myself.

But the OS I'm writing it on is nothing but my scratchpad offering me software to write the code in, and software to assemble/compile my code into executable instructions.

I've been told that I need to decide which OS I'm writing it on before I write it, but all I need is an assembler to produce flat binary, or a compiler to produce object code and a linker to link it into a flat binary .bin file.

Why do people say it matters which OS you make an OS on, when it doesn't?