As you say, it's largely the difference between serious work - i.e. "expert" tasks - and other things tech is used for. To call it a "hot trend" seems to overlook the reality that humans use technology for a lot more nowadays than just cognitively heavy work.
Even when the work itself is cognitively heavy, it is sometimes imperative that the cognitive load of the UI is low, since the user is actually using most of their cognition outside the computer screen: In the actual world with which they are interacting.
If you were at a doctor's office, would you rather have the doctor focusing their attention on listening to you or on complex data on the screen? - Possibly both, depends on the situation. But if the UI for data entry there demands heavy cognitive load, there is probably a cost for how present the doctor can be for listening and observing their patient.
That screenshot is quite old. We have improved our UI a lot since then. We have more screenshots in our website (www.debrispublishing.com) and you can also see a nice video here ( https://www.youtube.com/watch?list=PLfTMPTPc22sGNrm2rXt8kD-9...). But...as admax88q says, fintech apps are used to use large screens (and > 1).
For serious work, like investing, I want my software to use all my screen real estate and display as much information as is reasonably possible.