That 3.9 seconds from zero to 60 is probably one of the most controversial test numbers ever published in a car magazine - it's been the topic of discussion in many subsequent articles (even in C & D) over the years. Considering the skinny bias ply tires that were on the car, I think it was the result of an overeager thumb on the pushbutton of the hand held stop watch that was used for timing back then, but there also could have been some editorial creative license in writing the story.
I've been a full size Pontiac fan all my life, but I think it would be tough to get one to accelerate that fast even if you dropped it off a cliff.
edited to add: Looking at that out take photo above I see the 2+2 was equipped with what looks like NASCAR racing tires, which would have improved its chances quite a bit, but even so I think the 3.9 second number was probably embellished one way or another.