The xiao nrf52840 can be recovered with a double-reset press + erase the current.u2f file that appears on the mass storage drive that shows up when you double reset.
I discovered this by accident =D
it’s got to be a quick double press, triple press will put it back to running the bad code.
Nrf-connect appears to only show devices that have a J-link driver.
Because Seeed’s xiao modules don’t have the J-link firmware and use CMSIS-DAP instead, which is why they don’t show up in nrf-connect.
I am guessing the reason for this is J-link requires a license cost per board, and dap doesn’t, and Seeed didn’t want to charge an extra couple bucks on a 9$ product.