I think that kind of adjustment is usually the responsibility of the pollsters, with their likely voter models. I don't think FiveThirtyEight directly tries to also apply such an adjustment, because that would be at serious risk of overcorrecting.

Similarly, this year many pollsters have added level of education as a factor to their demographic weighting, to address a shortcoming in their 2016 performance. FiveThirtyEight consumes those poll numbers without adding their own layer of demographic adjustment.