Hi, thanks for the feedback.
Yes, I suspected what Gordon called "JS firmware" is what I would call the apps. I understand that it wouldn't be a problem to flash those from different platforms. I use Linux by the way.
I'm not sure where you found where it says it only works with USB devices though.
In the Web IDE when you go to Settings -> Flasher, it says it only works with boards with the Espruino USB bootloader.
In the command line version, I assumed it is the -f argument I need to use to upload a firmware, and this one explicitly says as well that it has to be a USB Espruino and Bluetooth is not supported.
All that taken together makes me think that it does not work with a Bangle. So are you saying you actually did update the Espruino firmware that way, although it says it doesn't work? Did you use the Flasher tool in the Web IDE for that?