What makes cake a Sponge Cake? And what doesn't?
You are asking for a firm definition where none exists. 

Baking is not mathematics, and it does not have a heap of literature where everybody has formally agreed to use the same terminology. Thus classification of baking goods is done into intuitive categories. This means it doesn't work by strictly defining where the border of a category lies (everything that has feature X is a member of the category "sponge cake", and everything that doesn't have feature X is not a member). Rather, it works by defining a center of the category, or a prototype for what a sponge cake should be like. The more a cake reminds one of the prototypical sponge cake, the more likely it is that this person will consider it to belong into the category of sponge cakes. And through social interaction, people will partly synchronize their ideas of what is considered "prototypical", and how close is "close enough" to still be considered part of the category. 

WIth that in mind, the best answer that can be given is to describe what a prototypical sponge cake is like. It has a specific texture, not that much different from other things called a "sponge", such as contemporary plastic sponges for dishwashing. It is basically a firmed-up foam, with many small, regular holes inside. It has a characteristic elasticity - not as brittle as a meringue, but not as soft as a marshmallow, it is rather in the middle of these two cooking extremes. It also has a characteristic type of being "moist" - it is not as dry as a baguette, but also not super moist like a brownie. It is chemically leavened, somewhat sweet, and vanilla flavored. (While there are variants like "chocolate sponge", they are not the category prototype). It is not noticeably oily in the mouth the way a chiffon cake can be. 

There is a chef named Ruhlmann who has tried to standardize some recipe types by giving a basic formula for creating a prototypical dish of each type. The book is called "Ratio", because the formulas are represented as the ratio of the ingredients used. For sponge cake, he gives a ratio of 1:1:1:1 flour:eggs:butter:sugar, and he requires that the cake uses the creaming method. A cake created with the same ratio, but using the muffin method, is given in the book as "pound cake". 

Following this ratio indeed produces a cake whose qualities are very close to what most people would call a prototypical sponge cake (with some cultural differences - I think that traditionally German and Austrian "bisquitboeden" are ligher on fat but else they are used interchangeably with Anglosaxon sponges). Making his pound cake (by mixing the same ingredients without creaming) results in a cake with a different, heavier texture with less regular holes. However, this does not preclude other recipes from creating a cake with a typical "sponge" texture without making use of a creaming method, or even from people finding the difference so insignificant that they would consider Ruhlman's pound cake to be a sponge cake too. 

So, it is not about any specific method, or ingredient, or ratio. It is about whether people who eat it will agree that they associate it with the term, based on whatever triggers their feeling of similarity.