Makes sense. The soda acts as an accelerator (it might soften emulsion a lot, so care would be needed), and the acids in the coffee relate to pyrogallol, pyrocatechol etc type developers. Tannins, caffeine, maybe even some related ones which are more like the regular reducing agents. You could probably fix using salt, but Lo-Salt would be a better bet than regular sodium chloride. I'm surprised at the clarity and the relatively short dev time and high sensitivity.