This board can programmed via DFU bootloader, using e.g. dfu-util.
To enter the DFU bootloader, double tap the reset (blue) button, or you can use machine.bootloader()
from the MicroPython REPL.
dfu-util -a 0 -d 2341:0368 -D build-ARDUINO_PORTENTA_C33/firmware.bin -Q # -s :leave