It’s indeed hardware scaling. The game itself can’t display other resolutions than what it was coded for. DOSBox will simply scale the image to the defined resolution, and apply hardware scaling.
You don’t really need this sdl function, because Magic Dosbox runs at your device’s screen resolution aumtomatically. The “Linear” filter supported by Magic Dosbox filter will do the same as “fullscreen=” with hardware scaling.
You can change the resolution of Magic Dosbox (or any other game/app) with Samsung’s Game Tuner tool, but this will simply reduce the image quality.
Btw, Game Tuner is one of the greatest tools Samsung have ever made. Ever wondered why an ordinary iPhone is “faster” than the S7? Because it only has a 720p screen. Just try Gfxbench on your S7 edge, with default 1440p, 1080p, and 720p. 😉