The SWD selection switch doesn't set the Thingy to be a Thingy:52 per se, it rather sets if the debug connector goes to the onboard nRF52840 or the nRF9160. You can see more on how it's used in the Thingy:91 user guide. Note that the nRF52840 is there mainly to implement BLE and NFC features on the Thingy:91, and won't be connected to all the sensors, etc. from the original Thingy:52. It is connected to a spare GPIO connector so that it can be used with various peripherals though.
