It's not quite clear from the article, but is this a stock Hyundai Nexo or was it modified to add more fuel tanks? The stock range of the car is 413 miles or 664km, so how exactly was this new record achieved?
It is called "hypermiling", you can do that with gas cars as well, by driving on a level road, minimal breaking and accelerating, plus driving in the engine's optimal RPM period in the highest gear possible. In short, the ideal conditions for the car.
Really difficult to be excited or impressed by these "records". Both Tesla and H2
The record is not the autonomy by itself, but is about getting the same autonomy for 35-70x less money and potentially 0Co2e emission.
You can make hydrogen from solar panels and store it until you need a refill vs. to charge your Tesla with solar, you'll need to surrender it in the middle of the day and wait for a full charge.