You are correct that the first “system” to focus on first is the “sealed system ” (a.k.a. “refrigeration system”).
The first things he mentions checking are just those easy data-gathering activities that you want to do at any job before you start digging a little deeper. After all, if you saw old, damaged door gaskets on both doors, there may be no need to go any further. So, even if the first system you want to evaluate is the sealed system, you still want to do that initial data gathering before moving the fridge out.
On the worksheet, after that easy “stop, look, and listen” step, he checks the compressor and condenser – this is analyzing the “sealed system” functionality.
Does that clarify things for you?