Shouldn't that answer then result in a "Invalid Question" to the original "Would you be a proper scientific skeptic if you were born in 500 CE?" question?

I mean, what you are saying here is that it isn't possible for you to have been born in 500 C.E., that you are a product of your genetics and environment and cannot be separated from those conditions that resulted in you. So the answer isn't "Yes" it is "That isn't a valid question."

I'm not saying I agree, especially since I think the initial question can be rephrased as "Given the population of humans born in 500 C.E. and the historical realities of the era, do you believe that any person born in this era could have been a proper scientific skeptic and given that, do you believe that you would have developed into one had your initial conditions been otherwise identical, or at least highly similar?" Making it personal (Would you be...) is just a way of conferring the weight of the statement, as it is assumed that the readers of LW all have brains capable of modelling hypothetical scenarios, even if those scenarios don't (or can't even in principle) match reality.

The question isn't asking if it is ACTUALLY possible for you to have been born in 500 CE, it is asking you to model the reality of someone in the first person as born in 500 CE and, taking into account what you know of the era, ask if you really think that someone with otherwise equivalent initial starting conditions would have grown into a proper scientific skeptic.

It's also shorter to just bring in the personal hypothetical, which helps.