The definition of the extents of a city is completely arbitrary. For example Manhattan is only part of a city, yet the authors don't even blink at treating it alongside cities that are located all around a hilly harbor with extensive sprawl (Sydney) or that constitute an entire country (Singapore).
sarnu
The author of the paper is also the author of a really useful python package for processing OSM data:
https://github.com/gboeing/osmnx
therockspush
Weird. I just bumped into this paper when I was trying to learn why Charlotte has such a confusing layout. I've never felt more lost.