Hi @pws
Here is the bootloader for XIAO: https://github.com/Seeed-Studio/ArduinoCore-samd/blob/master/bootloaders/XIAOM0/bootloader-XIAO_m0-v3.7.0-33-g90ff611-dirty.bin, you can use jlink to flash.
Please also refer to here: https://wiki.seeedstudio.com/Software-SWD/