Got an Adafruit RP2040 Adalogger Feather, and it previously had the Adafruit Wippersnapper offline firmware on it, which uses/creates a small FAT user-data partition to share with the OS.
I used the official flash nuke, then installed a new version of the firmware, but the file system partition showed up on the windows host with the old files still present (config-old.json etc). I repeated the flash nuke downloading it from the pi site instead of adafruit learn guide copy, with the same result.
Then knowing about Gadgetoid's alternative I used that (from https://github.com/Gadgetoid/pico-universal-flash-nuke ), and then installed the new firmware and it then showed the user file system had been correctly erased.

Why does the official one fail to nuke the board completely (leaves the fat partition)?