One key feature of APFS was the metadata location didn't overlap with any hfs+ metadata location, so the two file systems could co-exist on the drive. So I pres

One key feature of APFS was the metadata location didn't overlap with any hfs+ metadata location, so the two file systems could co-exist on the drive. So I presume they created the apfs metadata inside the hfs+ unallocated space, and so if it failed, hfs+ would happily overwrite the apfs metadata and continue on without any impact.