To determine if an observation is consistent with the spherical Earth model, we can create simulations to understand the expected result, and then see if they match the actual observation.
Flat-Earthers like to reject the results of simulation as being unreal, not real-world observation. In reality, the simulations are presented not to dispute their observation, but to demonstrate that their observation is consistent with expectation if Earth is a sphere.
We often witness flat-Earthers present their observation and claim that it somehow “proves” a flat Earth. Usually, it is due to their misunderstanding about physics or the geometries involved. One way to debunk their assertions is to show them a simulation of their observation. This way, we know how it would appear if Earth is a sphere 6371 km in radius. If the result of simulation matches with their observation, we can conclude that the observation is consistent with the spherical Earth model, and thus, does not disprove it.
Flat-Earthers would usually attempt to discredit the simulation by mentioning it is “just a simulation, not an actual observation,” as if the simulation was intended to dispute their observation. In the majority of the cases, nobody is trying to dismiss flat-Earthers’ observation as being faked. The simulations were created to show that their observation matches the expectation if Earth is a sphere, and, therefore, does not disprove spherical Earth.
Such their response is probably due to their own behavior if the reverse happens. If we were to show them an observation that proves spherical Earth, many of them would quickly respond by claiming it as fake.