Just to add on to this because this is a good response: the chart is showing it as a % change and Idaho is ranked 37th in population with 2 million. The #1 spot is California with around 40 million so for Idaho to see an increase of 1.5%, only 0.075% of Californians would need to move there.
Anecdotally, I know of cases of Californians moving to most of the states nearby, except the PNW. I don’t think it’s crazy to see that bump in Idaho as a result of the chuds and petite bourgeois exodus from the west coast neo lib regime.