Hi there,
Generally there’s not enough power output to DAMAGE anything , It just won’t work is all, Your thinking in terms of SWR and reflected power if there not a 1:1 ration or close. Like with high power HAM transmitters etc. Espressif documentation indicates and the Xiao has the proper matching Z so not an issue, it just will be weak or won’t go far range wise.
I wouldn’t modify the antenna that it comes with but you can place it inside the enclosure to limit the range to less than a meter.
HTH
GL 
PJ
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