Starting the init system is the task of the root filesystem or initrd, with any boot loader. Systemd-boot happily boot into any init system just fine, just like any other bootloader that can boot Linux will boot into systemd just fine.
Systemd-boot boots kernel images (with efi-loader code embedded) and only offers a menu to pick which kernel file to load. What makes systemd-boot interesting is that it does nothing more than that: It does not read random filesystems, it does not implement random encryption things, does not parse image files and complex theme configuration, … .