So put the adjective "walkable" in front. The question is the same. These things are unwalkable and therefore not able to be part of any route.

Same with your car: the places without roads are undrivable and therefore not available for routing. But with a car, the speeds are so much greater than bipedal motion, shortest vs. fastest is a viable question. When the routing system avoids obstacles, it's mostly going give you the shortest and fastest route. brador makes a good point about grade; with enough grade on the shortest route, it might not be the fastest.

No, these are all walkable.

I can't believe people are actually debating this. Did you really expect a check box 'jogging with backpack' mode?

It's a quick and dirty calculation that google is making about routing. Im pretty sure the google walking routes wont take you through marshes or thick forests.