Sure but assuming you're traveling from centre to centre, which is where population densities are highest, is a sound assumption. Otherwise you can always find spots where getting to the airport, train station, flixbus stop or whatever takes extremely long with one mode of transport over the other.

Doesn't distract from the point that long distances busses are very much not an alternative to rail (or planes for that matter) unless price is the deciding factor. And even the latter is questionable in many cases thanks to the 49€ ticket.