Some sooty buckskins do get mistaken for dark bay. The difference is the pale bits - on a dark bay, they're reddish. On a sooty buckskin, like Stan, they're yellowish. It's a very different colour to the fawn areas seen on dark bays. The real giveaway on Stan are those ears and throat - on a bay they'd be red, whereas Stan is a dark gold shade.
The other dun pointers he's missing are any form of a mask, zebra striping, and dorsal stripe. You wouldn't neccessarily see them all, but it's usual to see at least one on a dun.
This is Rhodri, who is a dark bay, so you can compare - you can see how the brown bits are red in comparison to Stan's lighter bits, especially around his throat. Sooty causes the addition of all the dark bits.