How is ground path formed between EMT and painted metal boxes? Is the paint conductive?
Surely the latter would be hit and miss.

Not at all. If properly tightened, it's guaranteed.
Since data can be fun: My multimeter reads open circuit/infinite resistance (more than 2 Mega-Ohms, I forget what the top of its range is, could be more than 20MΩ) for any distance between probes on the painted breaker box surface. It reads less than 1 Ohm between a galvanized cover screw on the painted breaker box and a junction box cover connected to a (galvanized) junction box connected to a fitting connected to EMT connected to a fitting connected to the main panel. The fitting in the main panel painted box was simply installed and tightened, no paint-scraping or the like.