Hi there,
I did a JLPCB for a couple and found the keep out area to be Directly along the back of the Xiao where my finger is in PIC , the first try I had signal propagation problems when i laid out components where the c3 is. after putting big (rev 3)ground plan around the bottom slightly around the sides It works easy 8 Meters indoors.
this one worked good until I connected anything to the PINS…no expansion there.
this one worked the best… note nothing but c3
most dev boards have the fin antenna over the edge also.
my 02.
HTH
GL 
PJ