It's not like the engine start procedure on one of those things is a turnkey solution. The amount of domain knowledge needed serves as a form of soft security.
The checklist is what you'd be reading, usually a laminated one-page card kept in a convenient spot. The documentation that the checklist refers to is the full "user manual" for the plane, with things like the quick reference handbook (QRH) that includes the step-by-step procedures covering what to do for every single warning light, emergency condition, etc. Also "documentation" here could also cover the mandatory things you have to have in the plane like the airworthiness certificate, registration, FCC radio license, insurance certificate, etc.
So I guess the short answer to your question is yes, in this context the documentation and checklist are not the same thing.
Right but my point is if you're premeditating the theft to the point where you've studied the manuals/practiced in flight simulators then a key isn't going to be the thing stopping you. I mean will that person realistically go "oh man, I've spent 100 hours learning how to pilot this thing, snuck into the plane, properly fueled the plane and did weight balancing but shit I don't have the keys, guess I'll just go home!". Nah, that guy is hot wiring that plane or stealing the keys on his way into the plane.
[0] https://flyuk.aero/assets/downloads/resources/checklists/UKV...