Is it updated because it is secure to install Steam in the distrobox container instead of layering it?
No, nothing to do with security. Locally layering steam on rpm-ostree systems causes dependency clashes. Using a distrobox avoids this.
What is the reason for this?
distrobox is useful when say building a package that only has build instructions for a specific distribution. Outside of that, it tends to be a bit of a crutch and a less secure option. flatpak provides sandboxing via bwrap, and brew has plans to add bwrap sandboxing for cli programs as well.
many breaches that secureblue itself has made in the walls
You’re calling the hardening toggles that we provide “breaches”? You’re annoyed at the added convenience for users?
GrapheneOS provides similar toggles for hardening… you can disable MTE, hardened_malloc, etc.
This seems like finding something to be annoyed about for the sake of it, and I respectfully ask you to not do that 
Including the need to install an unverified flatpak in order to do the first recommended way to install Steam.
Yeah, because there’s no official way to install Steam on Fedora or any distro besides Ubuntu for that matter. Take that up with Valve, not secureblue.