Are multiple browsers (safari, chrome, FF) on a PC/mobile treated as distinct entities in the device graph? if users use multiple browsers within a device to interact with a brand, it would make sense to make them part of the graph. However, probability of this scenario occurring is very dim, as most users have affinity towards one particular browser and they would never switch that loyalty.
Most of the purpose of the Device Graph is to combine what are typically tracked as individual visitors or devices into a single 'person'. Meaning in the stand along Adobe products yes each browser would be typically considered a unique visitor. But due to the device graph and combination of both it's directly mapped (aka the user logs in) or the algorithmic association it can deduplicate the amount of devices (browsers or physical devices like phones/tablets) into a single person.
I hope that helps,