When I first extracted and launched the game, the settings displayed my native 1920x1080, with full screen turned on. But it was actually running in a window at some unknown resolution. I swapped through the settings,  and there seem to be no widescreen resolutions available.

Given the game's aesthetics, is the aspect ratio limitation intentional and the game was just listing my native resolution falsely? Or is there some sort of bug I'm experiencing?

First start for me was also windowed at unknown rez, and the default rez option was listed as my monitor's native. Second start was fine, only 4:3 resolutions offered. Though you are able to set resolutions higher than your monitor can accomodate.