Smart physics point. What those guys and gals probably mean is that if you could get 400 lbs of force onto the head of a pin, then that may pierce the total support capacity of the floor. As you noted, the more you distribute it, the lower the pounds/square inch of pressure is.
Fat people also have the benefit of a generous surface area. =)
If that was their point, then I wouldn't say anything. But they were very adamant about not putting any +400 lb safe upstairs, which I just thought was hilarious!
And yes, fat people have a very generous surface area, but like I mentioned, if two or more fat people stood on one foot, they would risk falling to the floor based on this internet forum's "science!" :)