NavList:

Yes, an interesting new study! Thank you for sharing it. This has been reported before, starting more than a decade ago (*), for London cab drivers doing "The Knowledge", and I have been highly skeptical. But this looks like excellent new evidence and adds another group of drivers --ambulance drivers-- to the story.

Paul S, you wrote:
"the essence is that having to constantly navigate to a new location keeps the taxi driver's minds sharp."

It's not quite that. It's not about having a "sharp mind", though this is a popular idea with respect to Alzheimer's "do your crosswords! and you won't lose your mind!" But that's not it. It's the "mental map" that the drivers maintain and work with on a daily basis that seems to have an impact in some unknown fashion on brain health. It's not every generic type of mental activity, in other words. It's using that part of the brain that can visualize two left turns and a right turn half a mile ahead and then adjust that map in real-time when there's a fender-bender that blocks the current route. At least, that's what seems to be the common factor for these drivers: a live and change-able mental map.

You added optimistically:
"Which I would think likely applies equally to Celnav practitioners!"

Not a chance. First, the vast majority of celestial navigation practitioners are ritualistic, robotic agents. Many imagine themselves to be super-duper smart because "look, ma! I'm doing the speerical trigonomotry!!", but let's face it, the tools are designed and built for minimal intellectual activity. And most navigators follow painfully rote procedures. There are no left turns and right turns "on the spot" to avoid changes in whale traffic every three minutes when sailing across the Atlantic... There are, of course, some small number of navigation enthusiasts and practitioners who get much more from it, but even for them (us?), this is a small portion of the day. It's far removed from the instant mental navigation that ambulance drivers engage in, hour upon hour, day after day.

The article compares normal people giving up their daily navigation apps with the navigating problems faced by professional drivers. Quoting: "most follow the same well-worn routes, which do not require them to consult mental maps. 'It’s like saying that you could expect to derive the same benefits from walking half a mile a day as someone gets from running six miles a day.' " Yep! And I would call that an under-statement. A London cab driver is working long days, almost every day, applying the "Knowledge" and keeping an active updated mental map. Compare that with an average person commuting to work every day. It's more like walking half a mile a day (the commuter) versus running a marathon every day (the cab driver).

Frank Reed
* An article from 2013 in National Geographic also discusses this (probably paywalled, but they often have one free article per month or similar). Back then, the evidence was more speculative.