When creating scientific models of complex phenomena, we simplify things by reducing them to as simple a form as possible to make the calculations more feasible and less complex. Common simplifications include assuming objects as spheres, things exist in a vacuum, frictionless pulleys, massless ropes, and a flat Earth.
Flat-Earthers noticed that some scientific journals use a flat Earth in their models, and they use it as “evidence” the authors knew Earth is flat. In reality, it is just an assumption used in the models to simplify the calculation. It does not imply the Earth is flat.