This is true only for latitude, but not for longitude. In case of longitude the distance between two points 1 degree apart will depend on the latitude.
That's right people navigated using longitude and latitude for quite some time (mainly because is possible to measure longitude/latitude, for example using a chronometer and astronomical observations). The point is that angular measurements are more difficult to use than UTM grid. Maps can ease calculation of distances somehow (since they have a grid and a scale), but even with maps it is much easier to use UTM grid, since the UTM grid divides the map into into 1x1 km squares.