The author calculates this by searching for the term "AI" in the name and description of each startup's YC page. But presence of the term "AI" doesn't make a startup an "AI startup," so to speak. For example, I picked one startup at random, Topological, which is "developing physics-based foundation models for CAD optimization." Just because the company uses AI doesn't make it an "AI startup." AI is rapidly proving itself to be an extremely useful and workflow-changing tool, and many companies now have adopted it somewhere in their product without suddenly becoming AI companies.