I had a nightmare last week wherein I read a headline that systemd was writing its own kernel. When I woke up I realized it was a possibility, after all it has replaced GRUB. https://wiki.archlinux.org/title/Systemd-boot
And to be perfectly honest, it's nothing more than a philosophy - it's not some universal truth, e.g. a browser by definition is not doing "one small thing" and complex workloads are better organized by monolithic software to a certain degree.
How? This is really where it's basically a marketing fail.
Even your own link for system-boot shows that it is it's own rebranding of gummi-boot. It's not part of the init system, they just have an identically named project which has 100 utilities in it. It's dumb and it's community hostile.
It seems to me that the real differentiator is not the number of features but software release date :)
With unified kernel images there is no need for grub or any other bootloader anymore. And UKI simplifies boot configuration and helps improving security in some aspects.