Type2 loader entries are just UKIs[1]. I don't know what you are implying with "systemd-boot's automatic sourcing of UKIs isn't a BLS thing"? Any way I try to interpret it, neither would the Type1 entries fall under the definition of automatic sourcing (neither anything any other bootloader does).
[1]: https://uapi-group.org/specifications/specs/boot_loader_spec...