I got this!
The major magazine publishers were often working with prototype or pre-release copies sent over by the game developers. In the cartridge days, there wouldn't be any plastic shell housing the games, they would be the printed circuit boards with the appropriate ROM chips soldered in place. You generally couldn't play these versions on a standard consumer console, so the magazines would have special versions of the hardware (like the dev kits of today) that would allow them to play these versions of the games. These were intended to be hooked up to a computer monitor rather than a television, so it was easy for the magazines to use a monitor attached to a computer running screen capture software, and grab whatever images they needed, then clean them up using Photoshop.
The dev kits for portable systems did not include screens, so they too would be hooked up to a monitor just like the standard consoles, which is how they got such clean screen grabs of the games without any of the screen blur or other graphical issues seen, for example, on the GameBoy.