It’s right there in the installation options, and lined out for you.
Hassio doesn’t have an OS. Hassio is 2 docker containers (technically they run Alpine), but you are referring to HassOS.
If you installed hassio using the GENERIC LINUX INSTALL method lined out in the DOCUMENTATION, you can use Raspbian AND hassio.
The image you are referring to, is HassOS, which is a stripped down Linux distro that acts as a Docker host only. It’s not a general distro, and changes made to it, will not be retained.
The problem I had with the link you provided, is that I thought it was an installation guide of Home-Assistant, the one that you see on the browser. Hass.io menu item was for me just a menu item that is part of Home-Assistant as a whole.
Yes my friend, but for beginners you can’t expect from them that they know the difference between Home Assistant and Hass.io.
Anyway, we learn every day something new. Thank you for helping.