Guess this all depends on your definition of ATV. NOW an ATV is a machine thats operates on 4 low pressure tires. Back in the day before 4 wheeled machines became the norm (and even the only after they stopped making the 3 wheelers) ATV meant both......and even 3 wheelers before someone made a 4 wheeler.
In 1980 an ATV was a 3 wheeled machine.......in 2000 its a 4 wheeled machine.
The term ATV has meant different things......so I still consider a 3 wheeler an ATV.
Of course I cant argue the fact that the Argo qualifies.....seems it does to me, and many of the other early machines as well.