Thanks for your help, I really appreciate it.
I got it working again by installing the CircuitPython UF2, and then the Arduino IDE could connect to it again over the serial port (on a different port, which took a minute to realize).
But, for future reference, how does one getting a Xiao SAMD21 sketch in UF2 form? I tried compiling with the Arduino IDE 2.0, and the older 1.8 version, but either way I only got .bin and .elf binaries.
I found elf2uf2 utility for the RP2040, but it failed with “ERROR: Memory segment 00000000->0000fb04 is outside of valid address range for device.”
I assume that the Arduino IDEs are creating UF2 files as part of the upload process, but I have no idea where they are saving them. Not in the sketch folder, and not in the appdata\local\arduino folder, and I don’t know where else to look.