Yes, I did make that base myself. Roon OS is not based on any of those distributions. It was built from an understanding of what things are required to make Roon run and knowing the purpose and intention behind every piece of software running on UNIX-like operating systems and then designing a new minimal operating system.
You can see “from scratch” projects all over, but the most famous one is probably https://www.linuxfromscratch.org/ That said, Roon OS was not based on “Linux from Scratch” either.
It’s about as far from Manjaro as possible. Also, Manjaro is based on Arch, and Roon OS is pretty far from Arch too.
Roon OS has no swap.