I count 11 distinct colors, so strictly speaking one could call it 4 bit graphics (thus, 8 bit is closer than 16 bit).
Those images have way too many colors (11 colors each) to be practical on any 8-bit console, and the colors used are not appropriate to an 8-bit console's color gambit.
They do, in fact, very roughly approximate the "low-fi" style of 16-bit console games like Earthbound on the SNES, which include certain hallmarks of 8-bit-style art, like hyper-saturated colors and pure black outlines, but within the greater color depth afforded by a 16-bit console.
So, to be clear, does 8/16-bit refer to color depth (256-colors vs highcolor) or word size (NES vs SNES)?
Its almost always word size. CPU word size.
Most people who say "8-bit console" or "16-bit console" don't even know what a bit is. They just find it easier to categorize the consoles that way.