"Celtic" can mean different things depending on context — language, culture, genetics — and the time period matters a lot. The Celtic Index in this report measures how closely your DNA matches ancient individuals from Iron Age Central Europe (Hallstatt, La Tène, Gaul). These are often called Continental Celts.
Modern Celtic identity — as seen in Ireland, Scotland, and Wales — developed through many additional layers over time: Atlantic Bronze Age, early farmers, Vikings, and others. A 0% Celtic Index doesn't mean you lack modern Celtic roots. And a positive Celtic Index doesn't guarantee modern Celtic identity. It's one piece of a much larger ancestral picture.