Unknown freezer parts do not correspond to assembly diagram
jsotola was correct - after taking the entire freezer assembly apart, I found that the blue object was indeed just a bizarre clamp. The wires connected to it attached to the fan. I have no idea why it was clamped to the copper pipe.
After leaving a hairdryer running for ~30 minutes in the freezer, I actually found the two defrost thermostats mentioned in the specifications. They were located behind the evap coils and had been completely encased in ice. This isn't really relevant, but I think that is probably the most inconvenient place to mount the defrost thermostats.
Only divination can answer why an additional temperature sensor was added.