This is the firmware as-is on Xiaomao when it ships.
You need bossac to flash from the command line. You also need to reset the device just before flashing. My one-liner looks something like:
stty -F /dev/ttyACM0 1200 && sleep 2 && /opt/arduino-1.8.15/portable/packages/Seeeduino/tools/bossac/1.7.0-arduino3/bossac -i -d --port=ttyACM0 -U true -i -e -w -v JTAGscan.ino.bin -R