It's entirely supported by the article itself. The link you shared specifically says one in three Californians "say that their housing costs make them seriousl

It's entirely supported by the article itself. The link you shared specifically says one in three Californians "say that their housing costs make them seriously consider moving out of the state." I think you're just reading the wrong part of that page. (Why would the "most important issue for the governor" be the relevant statistic from that page?) There's also an infographic linked further down which repeats the statistic (with the words "startling conclusion.") "34% of Californians say they are considering moving out of the state due to housing costs." That's 13 million people. (It's the first line of the article.) The subset that goes from "considering" to actually leaving the state certainly accounts for the net 407,000-person difference in people who are leaving the state versus immigrating into it.