I was actually wondering about that when I was posting this. I even double checked the picture to make sure it was DOHC at the time.
When I really think about it, the FJ was introduced when there were no other DOHC motors on the Nissan lineup. The FJ was the first high production number motor to use that technology.
They probably didn’t fall into that nomenclature until they released the RB with DOHC.
Actually, there was the S30 (Inline 6 DOHC, 3.0L of the 70s) motor and the less known GR8 (Inline 6 DOHC of the 60s) motor before the FJ. The GR didn’t follow any standard nomenclature at all.
I guess they just kind of decided how to classify what it was in the mid 80s with the DOHC RB motor.