I'm not sure that it makes sense to group Boise with the other cities they mention (Austin, Denver, Portland, Indianapolis, etc.). Boise has less than half the population of Raleigh which I think is the next least populated of the cities that they mention.

In addition, while this is much more subjective, most of those cities seem to share a similar "cultural prestige"/influence as compared to other large cities like Columbus, Jacksonville, etc.

It seems to me that a more accurate description is that people are moving from the most expensive and culturally prestigious cities to slightly less expensive and culturally prestigious cities. It doesn't sound to me like they are moving to what people generally think of as "small cities".