For the most part, that surface contamination conduction is what you are addressing with conformal coating. It's also why there are complex tables that need to be deciphered to figure out what the minimum creepage distance is for a particular PCB for a particular application based on the contamination level. The inner layers of a PCB are effectively shielded from contamination which is one of the reasons clearance requirements are less on inner layers.
Arc over distance is all about whatever the dialectic of the material is. Open air conduction at room temperature and sea level isn't really the issue. The dialectic for air ends up with about 3MegaVolts/meter, or 2-3kV/mm. That's saying for 275V, you only need an air gap clearance of around 0.1mm(4mil) to keep it from arcing. Now if you used that as a clearance for a PCB on a board with the AD629, don't expect to get through any certification process, and you better be running in a clean room.
I'm totally cool with the AD629 requiring conformal coating to reach it's specd voltage rating, but they really should say that in the data sheet.