Nice bait-and-switch. The article isn't "The San Francisco Net Exodus", it's "The San Francisco Exodus". Just because there's no net outward migration doesn't mean that the outward migration occurring isn't notable. Note that I'm taking no position on whether the current overall migration situation is good, bad, or insignificant, just that migration of a certain part of a population (in this case, income level) can be worth studying/discussing, even if they're more than replaced by others.
As an example, if migration between Canada and the US increased a thousandfold, but the net migration stayed the same, you can be damn sure I would think it was interesting and worth looking at. Big changes in demographics and migration patterns are always interesting (for better or worse), and net migration is never the whole picture.