Yeah, that's not really how graphics hardware worked back then. 8-bit in this context doesn't refer to using 8 bits per pixel. That gives you 256 colors, which came later; that's a late 80s/90s thing. 8-bit here is about the CPU, e.g. 6502 and variants.

To use the C64 as an example, with the VIC-II chip there was only one bit per pixel, but you had various modifiers that would allow you to get more colors, with various restraints. e.g., you could effectively cut the horizontal resolution in half, so the next pixel would combine with the current pixel, for a total of 4 color possibilities, however 3 of the colors would be shared for the entire screen, but the 4th would depend on what color you chose for each 8x8 character (1,000 characters on a 320x200 screen), BUT only half of the 16 colors worked for multicolor mode, and the other half resulted in "high res" 2-color mode, so you could mix the two freely by being careful about what colors you used. By contrast, Atari 8-bit hardware had more colors, but not this kind of per-character flexibility. That's leaving out sprites, not to mention raster interrupts and other hacks.

Despite the differences there's kind of an 8-bit "vibe" for hardware of this era. Most modern so-called "8 bit" games take a lot of liberties and don't really nail that vibe.

Outside of Gog's blurb, the game description does describe it accurately:

"Relive the glory days of classic gaming with retro-inspired 16-bit physics"
OMG, now marketing gibberish is including not only graphics but physics as well.

That's not gibberish. That's admitting that the physics are bad. Weird choice, but good of them to admit it.

Big "Bastard!" vibes on the cover (where are you, SJWs? This is a faithful recreation of late 80s sexy-cringe cover style, it's not progressive!)

The game itself looks like a love letter to the first Castlevanias to me.

Promoters should not try to be over-technical, ending up unwillingly comedic: "16 bit" and "8 bit" look like SEO words in this presentation, "16 bit" being simply a graphic style here (but here we're closer to NES/8bit graphics with a modern twist) and "16 bit physics" not even existing as a term.
Of course 16 bit processors can deal with a bit of physics..arcade physics that is, another way of saying this is not a simulator. Is Jonathan Joestar secretly Richter Belmont?

Very NES but looks cool.

It reminded me of Bloodstained Curse of the Moon. Speaking of Curse of the Moon... Dear GOG, bring them both here. ;)