I tried the reboot many times. It may be as you say, I should have tried more, however I was able to resurrect the 3 XIAO boards by flashing a hex file from an arduino Zero compiled in linux. After that the XIAO boards were recognized as Arduino Zero, but using PlatformIO I flashed a sketch for XIAO and it updated the bootloader correctly. The USB ports on my computer usually give problems, so they may be complicit in the problem. Thanks for your help, without your repair guide I wouldn’t have dug deep enough until I solved the problem. 
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