Not affiliated with the company in the article, but this isn't necessarily a symptom of ageism.
Early-stage startups generally pay toward the lower end of the SF engineer pay scale (<$150k). At that salary, it can be hard to find local experienced candidates. For $150k/year it's impossible to buy a house and put down roots, so people at a stage in life that value that (which is a significant percentage of the over-30 demographic) will often move out of the Bay Area. You'll make a bit less money in Denver, Austin, Boise or Salt Lake City, but you'll be able to buy a house with a yard and raise a family if that's your thing.
Of course, there are still experienced engineers around, but they're disproportionately making $300k+, which can still buy you a decent quality of life in the Bay Area.