my BCx would alert bark regardless of whether people were home or not, I do not think he has the physical confidence to take direct action though, I suspect he would be extremely anxious if I were threatened but I doubt he would bite - growl maybe.
My other dog has the typical staffy attitude of 'a stranger is just a friend I haven't met yet - especially if they might give me food', she doesn't alert bark and I have no idea if she would react at all if I were threatened, maybe she would surprise me?
The dogs I grew up with were a different kettle of fish, one was a rescue bully x ACD former pig hunting dog and was very protective of all the family, other dogs included - but would take action against a perceived threat in total silence. The other was a GSD, highly adaptable in his approach, he was a very special boy (and took his responsibilities very seriously). I have no doubt that those dogs would have not hesitated to protect me against any threat.