Things I like
For Alaska and Hawaii the data relative to years before 1960 was missing. I really liked the way the authors dealt with this problem and I believe the resulting visualization was well studied.
In the map chart they put a sentence saying that data was missing for Alaska and Hawaii when the user selected 1900 or 1950; in the aluvial graph they simply started from 1960, what is noticeable is that they did not rescale the graph to make use of the all space, this lets the user compare even Alaska with other countries because they keep the same scale.
Although direct comparison is not really possible with the application.
The colors of the legend are meaningful, light, and divergent; the number of categories is 5 so the user has no problem identifying the clusters. This is exactly how the choice of categories should be done.
The aluvial chart to represent incoming and outcoming flow from a state is just RIGHT. It gives the user the possibility to extract trends over times, see patterns between different states and studying the variations in the composition of the population.