Paint manufacturers like to advertise their products as "one coat coverage" but they fail to emphasize the many variables that determine how well a paint film covers.
One of the factors is the amount of paint applied to the surface. By recommending a 3/8-1/2" nap roller, the manufacturer is at least assured that a decent amount of paint will end up on the surface, thereby bolstering their dubious "one coat" boast. Feel free to use a foam roller if you wish (I'm not a fan); if you are concerned about coverage use a thick nap roller then go over it with a high quality brush, low-nap or foam roller.
I would question why you are using primer as most steel doors come pre-primed. You do not need to prime previously painted or primed surfaces except for certain circumstances with specialty coatings or where you have an issue with the surface.