They were assuming that it was a Round Earth Radar Test, as opposed to a Flat Earth Radar Test. They were comparing the Round Earth coordinate devices (GPS) to the Radar Test values, without knowing which shape of the earth they were on. The distance of a mile would measure differently on a Flat Earth vs using a Round Earth lat/lon coordinate system.
TL;DR: there's no such thing as a "flat earth mile" vs a "round earth mile". The mile is the distance covered by light traveling in a vacuum in the time span of 49,347.828 "ticks" of the atomic clock.
Long version: The mile was standardised at exactly 1,609.344 metres by international agreement in 1959.
So what is a meter? Is it defined by a round earth feature? Well, it used to be: originally the meter was one ten-millionth of the distance between the North Pole and the Equator. Several refinements later, the meter is now defined in terms of the second and the speed of light: the metre is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.
So what is a second? Maybe it is defined by a round earth feature? No, but it briefly was, between periods before and after where it was not. From antiquity, time was measured against the day/night cycle, which is observational rather than theoretical. (By which I mean that no matter what shape you think the earth is, we can all agree on the observed timing of the exact moment the sun is/appears to be at its zenith.) The second became measurable with the advent of mechanical timekeeping (clocks) accurate enough to keep good time, and in the 1670s the spread of the grandfather clock effectively defined the second as 1/60th of the minute. In 1862 the second was formally defined as 1/86,400 of the mean solar day, which was carried into the metric system when it was adopted. Then, for nine brief years beginning in 1956, the second became tied to the round earth when it was defined as the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time. I say "tied to the round earth" because nobody had measured the length of that year, it was calculated from heliocentric theory and the observed length of the contemporary year. By this point, however, it was well known that the earth's rotation is not a constant over time, and is thus a poor standard against which to measure time. So the atomic clock was born, and in 1967 the second was defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom