What appears to be happening is that for some reason the game is getting the value of "VRAM" assigned to the integrated GPU (which, according to the Mac System Info report, is indeed 1.5 GB) rather than the discrete. I've tried forcing the discrete GPU to be active before the game starts up rather than letting it switch over automatically, and that makes no difference.
0.000 2018-03-01 09:04:19; Factorio 0.16.27 (build 35809, mac, steam)
0.000 Operating system: macOS 10.13.3
0.001 Program arguments: "/Users/dmsilev/Library/Application Support/Steam/steamapps/common/Factorio/factorio.app"
0.001 Read data path: /Users/dmsilev/Library/Application Support/Steam/SteamApps/common/Factorio/factorio.app/Contents/data
0.001 Write data path: /Users/dmsilev/Library/Application Support/factorio [377703/953284MB]
0.001 Binaries path: /Users/dmsilev/Library/Application Support/Steam/SteamApps/common
0.041 System info: [CPU: Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz, 8 cores, RAM: 16384 MB]
0.067 Display options: [FullScreen: 1] [VSync: 1] [UIScale: system (100.0%)] [MultiSampling: OFF] [Screen: 255] [Lang: en]
0.068 Available display adapters: 1
0.068 [0]: resolution 1680x1050px at [0,0]
0.068 Create display on adapter 0. Size 1280x720 at position [190, 147].
0.701 [OpenGL] glClear in ogl_clear:0: GL_INVALID_FRAMEBUFFER_OPERATION
0.726 Initialised OpenGL:[0] AMD Radeon Pro 460 OpenGL Engine; driver: 2.1 ATI-1.60.26
0.726 Dedicated video memory size 1536 MB