The object will end up transparent but I don't know how to explain it.
This is a compositing issue I have tried using a mix node and aplha over node with different FAC values and get a product worse or the same. The file google drive
In a properly encoded RGBA buffer, the RGB represents the quantity of emission of a given pixel "window", and the A represents the degree of occlusion of the geometry.
The proper approach here is to simply take your rendered image and apply an Alpha Over your background, at a "full" 1.0 factor.