On floppy disks, the original boot sector is moved to cylinder 0, head 1, sector 3, which is is the last directory sector on 360 Kb disks; the author presumably believed that it was "safe" to overwrite.
The original boot sector is moved to cylinder 0. This was on floppy disks. Cylinder 0 is head 1. Head 1 is sector 3. Cylinder 0 sector 3 is is the last directory sector on 360 Kb disks; the author presumably believed that it was "safe" to overwrite. Cylinder 0 is head 1.