There are many thoughts on how to exactly define a banyan fig.

There are some figs commonly called "Banyan Figs", such as Ficus benghalensis - the "Indian Banyan" and Ficus microcarpa - the "Chinese Banyan". These trees although called banyan do not necesarily grow in the banyan form. It is only when high humidity persists for weeks or months that these figs can grow aerial roots and form banyan shapes. In drier areas they have normal single trunks and no aerials or secondary trunks.

In my mind a banyan is defined by aerial roots. If the bonsai design has no aerial roots it is not a banyan. The shape of the canopy is also not important in calling the tree a banyan.

Raintree with tropical canopy but no aerials = not a banyan

Many tropical trees growing out in the open fields will show wide-spreading canopies and dense foliage. While these are a very characteristic feature of the tropical landscape they are not in my opinion banyans. Add some aerial roots and voila they become a banyan style.

Ficus salicaria, Willow Leaf, banyan style,