While il88pp's statement is misleading, for a long time there was demonstrable evidence that Intel CPUs had better performance in the areas that mattered to FSX (Intel had much better single core performance while AMD started going towards multiple cores), so were the best ones to buy if you were looking for the best performance. This didn't mean AMD CPUs didn't work, just that you would likely be lowering the settings a bit further to get the sim running smoothly. With more recent CPUs, the difference has dropped with CPUs from both sides being more powerful than their predecessors, and both sides are able to run the sim better.
Similarly on the GPU front, there were demonstrable cases around the time FSX came out where an AMD card would take a performance hit while the Nvidia ones wouldn't. Flying through clouds was one situation. Nvidia also provided specific driver updates for FSX long after AMD stopped caring about the sim. AMD has long since changed their GPU architecture, removing those issues with their GPUs.
It is not uncommon for games and applications to run better on one CPU or GPU over another, and can be more than just opinions trying to push and agenda. FSX will run fine on most Windows computers these days with the right settings.