The Roman empire in the West was rather slow to collapse, even the fact that it survived the crisis of the third century [0] speaks more to its resilience than fragility. As late as 460, Western Roman commanders were able to subdue hostile barbarians and reattach their territories to the Empire proper [1].
The Roman empire in the East, as you notice, survived for another thousand years.
If the author wanted an example of an empire that collapsed really fast, it would be the USSR. That was indeed rather fast. In 1985, Moscow controlled not just the USSR itself, but several important European satellite states east of the Elbe, plus it held a lot of sway in the developing world. Six years later, the empire was gone. Not even Western Kremlinologists expected such a fast unraveling of the Soviet system.
[0] https://en.wikipedia.org/wiki/Crisis_of_the_Third_Century