ipodpatcher isn't "completely replacing" the apple firmware there, it's just writing a rockbox image where it would normally write the bootloader.
No, my description is correct (if by "Apple firmware" you are referring to the part of the firmware on disk, not the flash-based Apple bootloader/disk mode/diagnostics mode).
Normally, ipodpatcher (and therefore rbutil) will append the Rockbox bootloader to the end of the Apple firmware on disk, and change the "entry point" (stored in the header to that firmware) to point to the start of our bootloader, instead of the start of the Apple firmware.
What the "-wf" option to ipodpatcher does is to completely replace that Apple firmware image with the Rockbox file (bootloader or main Rockbox).
i.e. a virgin ipod looks like:
[OSOS]
[AUPD]
[RSRC]
with a standard Rockbox install, it will look like
[OSOS and Rockbox bootloader]
[AUPD]
[RSRC]
and with a "-wf" install of the Rockbox bootloader or Rockbox itself:
[Rockbox bootloader or Rockbox itself]
[AUPD]
[RSRC]
Where "OSOS" is the main Apple firmware, "AUPD" is the image that flashes new ROM contents, and "RSRC" is the resource block (only used on the Nano and Video).