They are not as easily calculated.
MPG, take the mileage from your trip odometer and divide by how much gas you put in the tank.
L/100KM, take your MPG and then divide by 235.2
If you enough money for 5 gallons of fuel how far can you go? MPGx5, L/100KM who knows.
I'll take that to mean if you had enough money for 5 liters of fuel how far can you go so you're not adding the conversion to metric. If my car gets 10 l/100 km, it's obvious I can go 50 km.
But that's not what the article is about. It's about people trying to compare the efficiency of cars when shopping. As I've shown, a volume/distance measurement is better for that purpose than distance/volume.
So, you buy as much gas as you can with the money in your pocket, then pick a destination you can reach. "Hey! I found 20 bucks on the floorboard! Lets go to Albuquerque!"
On the other hand, if you travel 100 miles round trip for work, you know exactly how much gas you need if you know gallons/100miles.