That temp controller is not very good at all, 7 Degree offset and no decimal points in the reading or offset calibration value.
That's likely just the display output rounding.
You are correct since the ADC has 0.25 degrees resolution after reading the datasheet.
After looking carefully on the video, I think they might have made the mistake of not using a proper thermocouple-wire from the connector to the circuit board inside the box. The wires from the connector is soldered to the board, but I have never managed to solder the Nickel based alloy used for Type-K thermocouples, the solder does not wet this alloy. If this is the case, you can just replace that wire and possibly gain several degrees of better accuracy, and you can even leave the box on top of the oven without any problems
I already posted the datasheet in the video description.
I completely missed that, thanks!
But for the same amount of money (123 Euro) you can get much better performance, for example this industrial miniature thermocouple controller (which I have used as an oven controller previously).
http://www.omega.com/pptst/CN7500.html Cost 70 Euro (and then you buy a solid state relay and a fancy box for the remaining 50 Eur you have saved)
This can handle all thermocouples types (even other signals such as 10 mV per Celsius and PT100) from -273 to +1800 Celsius with 0.25% accuracy out of the box without calibration (better with calibration of course). It has dual temperature display and up to 64 different ramp/soak steps with build in extra relays for customizable alarms. Omega also have free software for changing parameters and proper logging/graphing capability for more than 200 devices on the same RS232 or RS485 cable if you ever would like to expand your toaster oven reflow capacity.