The air defenses weren't that great. They could knock down some B52s but the majority could deliver their payload and return. The Wikipedia article on Lineback

The air defenses weren't that great. They could knock down some B52s but the majority could deliver their payload and return. The Wikipedia article on Linebacks II says the US air force targeted military targets and power plants. Around 1700 people died. A true carpet bombing of a city like Hanoi would result in a lot more than that.