Step 5 compared every narrowed pack with verify and never the --split parts with the full pack, so a part writing a path the full pack settles passed every check. check_split_packs builds each platform's parts in memory and compares their union with the full pack, path for path and byte for byte.
Each part was built alone: the Other_Cores part never saw the platform's destinations and each part had its own case fold. RetroDECK's parts wrote bios/SGB1.sfc/ members beside the file bios/SGB1.sfc, RetroBat's wrote 21 openmsx/ twins of openMSX/ files, and a path two systems declare got each system's file. The full pack is now built in memory first and a part writes a declared file or core extra only where the full pack writes the same bytes.
Abdessamad Derraz