Grounding solar controller
I agree with you, there seems to be a bit of confusion.

But I think what they mean is, that in the case where the batter "-" terminal is connected to the chassis, thus grounded, then this should be the only point of grounding in the system.

That means the solar panel and the load(s) you have on the controller are not allowed to have electrical contact with the chassis, with neither of their two terminals.

So the conclusion is: I think you can indeed use the circuit you have in mind, with the battery negative connector grounded, but with the condition that the neither the panel nor the loads are grounded (have electrical contact with the car chassis).