What I understood from reading the post-mortem was that GoI did undergo a semi-forced (by the publisher) art shift midway through development, it used to be a lot grittier, with a more 'cluttered' and complicated art style, as opposed to art deco which is more clean, symmetrical, predictable, straight lines, etc. The artwork from kickstarter page may have been produced after the art shift. That's a possible doylist explanation.
My preferred in-universe (watsonian) explanation is that the ruined buildings were likely built before the apocalypse, back when industry was still well-developed and people believed in the power of scientific and technological progress. That was a stark contrast to modern times, with most people just struggling to get by, and the ones with power are trying to scavenge wondrous technology from the ruins for themselves, instead of improving life for everyone. This difference in attitudes is reflected in the designs of ships and buildings.
The port city in the painting is an exception, it's obviously located in one of the more well-preserved parts of world. Likely chaladon, judging by the amount of vegetation (most other places aren't nearly as lush because climate change).
p.s. if anything, my issue with that picture is 'strong angular forms' being a bad idea. The pyramidion is 'strong and angular' incarnate, but it fits into the game quite well.
