Whether or not a dog will bite someone depends on so many things.
Whether or not a dog will bite someone and stay on that bite until commanded to let go is something else to consider.
Again, it comes down to...
one's definition of "guard" or "protect".
Does guard dog mean...
a dog that will bark when a stranger enters the property
a dog that will bite someone who enters their property regardless of intent (no discretion)
a dog that will only bite if a physical threat is being made towards it or it's owner(s)
a dog that will bite and stay committed to the bite until commanded to let go even if if the intruder is fighting back, attempting to hurt the dog etc.
Something else to consider....
If you were in a bar and someone walked up to you and started punching you for no reason, but you just stood there and took it or tried to walk away, would you consider the person who attacked you to be a "tough guy"? Obviously not because you never attempted to fight back. Most people, when encountered with an aggressive dog are going to either freeze in their tracks or back away so of course the dog is going to appear confident. With many dogs, you will see that confidence fade pretty quickly when they actually encounter a person that will come forward at them with aggression (taste of their own medicine so to speak).
See what I mean when I say it depends on your expectations or definitions?
does "protect" or "guard" = strong character? Not necessarily.