The referenced document seems outdated, and I’m not sure if the described workflow is the most appropriate and convenient for creating derived images of OSTree based systems.
If your goal is to make a custom Fedora Silverblue image, you’ll likely have to follow the container native OSTree (bootc) approach which is not yet officially supported and documented, at least for Atomic Desktops.
Yes, it is, but the exact answer depends on the meaning of “OS for me” in your case. As @hricky already pointed out, bootc technology is “cooking” now. With that said, depending on your goals you might still find it usable. I used https://blue-build.org/ to build a variant for me off this base image - Package silverblue-main · GitHub. This is not an ISO builder, I installed stock Silverblue, then re-based to my image and it brought everything I needed. This variant represents my “OS for me”, as it is usable for my “average person” use case. For better supported bootc Fedora Atomic-based OS (for users, developers and gamers), please check the https://universal-blue.org/ project. You can copy those projects and customize it even further.
This can be achieved with blue-build.org, minus ISO online on course. They have instructions on how to build it locally and later you can upload the ISO to a could of your choice to be shared.
You can also share couple of re-base commands which can be used after stock Fedora Silverblue installed. There is no real need for ISO. It just takes ~20 mins, 2 commands, 2 reboots to get your OS delivered.