Q: 

Is this true?

http://www.plusp.com/classroom/lesson12.php (This article describes a number of very common cases where people found strangers in their house, yet were not legally able to employ deadly force on those strangers.)

If someone has entered my home uninvited, I would consider it a legitimate threat.
 

A:

Actually the courts would probably not consider a stranger in your house, just by itself, to be a "legitimate threat".

That article was right on the money. Be very careful here. The article described some good examples of how you would not be legally correct in shooting those people.

However, if you have your doors locked (and there are no other keys available to anyone) and your windows locked and someone HAS to bust their way in, that's a different story. That's usually described as a "tumultuous" entry and offers you more protection.

The base test is that any reasonable person would be afraid of this invader because it was apparent that they (the homeowner) were in imminent danger of death or serious injury.

Be prudent.  Get your family into a safe room or a safe zone where you can defend the zone if the stranger in the house has evil intentions.  Call 9-1-1 and tell the dispatcher exactly what's going on and let them know if you are armed. 

Once you've taken up a defensive position, resist the temptation to "clear" your house by yourself.  The cops are on the way, you have yourself and your family safe.  Don't give up your cover to go out and try to apprehend the subject yourself.  It is a very difficult task and one that most police officers don't do alone.  They wait for a backup officer so they can clear the house as a team.  (There's a saying in the tactical and law enforcement business, "Teams survive while individuals die").

Once the cops arrive, stay cool and lower your weapon.  Let them deal with the intruder.