Constant BLS_ENTRIES_DIR 

const BLS_ENTRIES_DIR: &str = "/boot/loader/entries";

Directory where BLS-compatible bootloaders expect Type 1 boot entries.

Its presence says nothing about the bootloader on its own: EFI systems have it too, and GRUB reads the same entries via blscfg. It is only consulted when there are no EFI variables to inspect, and then only together with GRUB_DIRS to tell a BLS-native bootloader apart from GRUB (see classify_bootloader).