It is common practice for engineers to state their assumptions when presenting documents for review. There is absolutely nothing profound about a statement such as, "assumes a rigid body over a flat, non-rotating earth."

It actually shows that the engineer believes that the earth is round and moving because he's stating that he's ignoring those variables in his calculations.

Math is hard. At these levels, performed in the 60's with slide rulers, it is incredibly hard. Engineers use shortcuts to make calculations easier and reduce errors. Trying to account for the changes in air friction because of flex of an airframe under the varying conditions of wind, correolis effect, turbulence, diminishing fuel load, non-standard temperatures or pressure is unnecessary. That data shows up during flight testing.

This is a common, standard engineering practice and is not 'proof of a flat earth'.