I think the general public holds the belief that if their rescued dog is skittish over a certain object or situation that they've been abused. So often this just isn't the case. Of course with rescued or adopted dogs, most of the time we just don't know what their lives were like in the past. And yes, I'm sure a good many of them have been abused. But why do we automatically assume the worst? I'm guilty of it too. As a kid we adopted a wonderful shepherd/collie mix from the shelter. She was a year old. She was big, wild, and so it seemed, absolutely thrilled to be going home with us. She had her fears...the garden hose for one. We all thought that somebody must have really abused her with one. Thinking back on it....that probably wasn't the case. She loved people, and I have to think that if someone had abused her, she would have been a lot more weary of strangers than she was. Assuming abuse and feeling sorry for your dog isn't ever going to help them overcome their fears. What happened in their past is over. Dogs don't harbor resentment or sit around feeling like victim's. They move on.
However, let's look at the undersocialization aspect for a moment. The opportunity for socialization is early and short. Puppies are primed for socialization between 3 and 12 weeks. Most puppies go to their new homes around 8 weeks of age. That gives you 4 weeks to get them out and expose them to the world...and I mean every novel thing or person you can think of. So, if you're adopting an older puppy or dog, their early life may not have included all this great exposure to the world. Maybe they were strays, or in a shelter, or worse yet a puppy mill, or born to some back yard breeder that only cared about the money they would bring in. They may never have had a single bad thing happen to them. (although not being at all socialized is a very bad thing). They are still going to be fearful and shy and spooked by new objects, people, and circumstances. Why...because they've not learned the most important thing that early socialization teaches...how to roll with the punches, how to recover from momentarily being frightened by something new (I call that "bounce back"). Sure, they store the memory of having seen a man wearing a funny hat before, but I think it's really all about gaining the confidence and independence to feel that "unexpected things happen in my world, but I'm fine with it".
Whichever past your dog has overcome, don't feel sorry for him. On the other hand don't ever force him to interact with anyone or anything that truly frightens him. Slowly get him out and about where he can get some experience with the world and always praise and reward his calm, brave behaviors. When he knows you've got his back, he'll start to feel more confident. And there's nothing wrong with removing him from a scarry situation or calmly comforting him if he is afraid!