Earth’s diameter is around 13000km, if there is an inner core with a 600 km width, you will find this core around 6000km below your feet, which is more than 1600 km below your feet (as the article correctly stated…)
[1] https://en.wikipedia.org/wiki/Core%E2%80%93mantle_boundary
644 km is 400,000 miles (within 0.04% error).