I am so sorry for this family and their loss. Losing a child is one of the hardest things a person can deal with. 
First CO is .97 the weight of air. So the person quoted here is correct but it isn't much lighter than air so it doesn't rapidly float up to the ceiling. The air would need to be very stagnant and have time for CO to stratify to the top of the air column. It can be anywhere in the house. The article mentions a boiler so the CO source was probably flue gasses escaping in the mechanical room or garage. There is an air gap in most flues that would allow the flue gas out if the event of a obstruction.
Second: The article quotes a person saying they are surprised that a newer home would have this problem. One of the issues is that newer homes are way better sealed so they are more likely to keep CO inside the home in the event of a CO leak. 
The level of CO in this house was above NIOSH IDLH.