So I found every add-on that can access /backup folder has a mapping for "backup:rw" in its docker config. For instance: Samba share and SSH & Web Terminal
Also found that “real” backup folder in raspbian is /usr/share/hassio/backup, but the installation method I used doesn’t bind /backup, therefore homeassistant container -and core components- cannot access it.
I opened an issue to ask: https://github.com/home-assistant/hassio-build/issues/129