The extension cord creates a few problems. The safety problem relates to the resistance added by the extension cord. Resistance causes a voltage drop (think of that as a pressure drop) so to get the required power, more current is drawn. That means that a system rated at 16 amps is actually pulling more - on a circuit that should only be loaded to 16 amps.
And once this starts it gets rapidly worse...
The amount of drop depends on length of the wire, thickness of the wire and quality of connections among other things..
The other issue to consider is liability. You have electrician installed wiring. You have a ULC approved appliance. You have them connected with, what an adjuster would see, as a home made bandaid.
Good point!
Those are the reasons for the warning in the manual.
Rob!