Other commenters say it's historical are, well, mostly right, but there is a distinct advantage to knots: a knot is 1 NM per hour, and 1 NM was historically def

Other commenters say it's historical are, well, mostly right, but there is a distinct advantage to knots: a knot is 1 NM per hour, and 1 NM was historically defined as one arc minute latitude. So if you are looking at a chart and doing calculations by hand or in your head, NM and kts are very convenient. It lets you mostly do chart -> distance -> time and fuel measurements/calculations in your head. Of course this is not perfectly accurate in several ways but given chart projections it's usually Close Enough. The speeds given in the article are a little ambiguous in the details, the number given in the text is probably KIAS (but maybe not?) and the numbers from FlightAware will be ground speed.