Earlier quoted context omitted.
The quoted paragraph clearly states that the lower-emissions route will only be given by default when there are multiple routes with a similar ETA. So it will never tell you to take a route that takes longer just to reduce your emissions, without also telling you about the shorter route with higher emissions. Since you have seemingly already abandoned google maps due to their activism before reading this story, what…
How is the fastest route not the least carbon footprint?
The faster you go, the more your fuel efficiency decreases. For the most part this is due to wind resistance, because there is a velocity^2 argument there(ie if you double your velocity, you quadruple your wind resistance). Your engine is also designed to operate more efficiently at certain speeds but that is less directly quantifiable than wind resistance.
So you might be able to get 40MPG while doing a steady 40 mph, but only get 20MPG when doing 80mph.
So if you need to go 40 miles, if you can do it at 80mph it will take you 30 minutes, and will cost you 2 gallons of fuel. If you do it at 40mph it will take you 1 hour and cost you 1 gallon of fuel. So in this contrived example the longer trip has a lower carbon footprint.
In the real world, there are rarely two parallel roads going exactly the same place but just with a different speed limit. Usually you need to drive out of your way to get onto a highway to make your trip go faster. On the other hand driving directly at a slower speed usually entails more starts/stops at traffic lights which eats into your fuel efficiency. Google is probably in a good place to answer the question of how fuel efficient is a given route because they have so much data about average speeds, accelerations/decelerations required/ etc.