Adafruit nRF52840 Feather

the espruino_2v09.134_nrf52840.hex is most probably full hex file with softdevice included. to build --application dfu zip you should take the intermediate file called espruino_2v09.134_nrf52840.app_hex which is only the application.

Also the softdevice version must match what adafruit bootloader currently uses. Or if it doesn't use any softdevice (the newest one can be build like that?) you probably need to flash matching softdevice too - make another zip package with --softdevice softdevice.hex , not --application and flash both zips separately. see the end of espruino build output for file names, you should see mergehex running.

or alternatively it may be easier to convert full hex file you have to uf2 instead of dfu zip and flash that over usb as per https://github.com/adafruit/Adafruit_nRF­52_Bootloader#making-your-own-uf2