I know explaining this to you is probably a waste of time, but when the paper abstract says it is a linear model of aircraft flight based on a flat, nonrotating Earth, that does not in any way mean that the authors believe that the actual Earth has those properties. Those are mathematical simplifications to reduce the complexity of the model, which approximate the local conditions that the simulated aircraft is flying in well enough.

All models are approximations of reality. Those particular properties of reality which are not relevant to the object of study can be neglected. As another example, a scientific paper might create a model of a liquid flowing around a perfectly cylindrical object. Of course there is no such thing as "perfectly" cylindrical when you zoom into something at the microscopic or atomic level, as it will have imperfections, and not be perfectly smooth. Those imperfections are small enough to not be modeled and have a negligible impact on the macroscopic behavior of the fluid flow.