Another example, but more extreme.

This is the same scene, but I've enclosed the two primitives (the cube and the sphere) with pure white walls and a ceiling surrounding them. (I moved the camera and changed focal length, too, as part of my playing about with the scene but I doubt that this is a factor here.) I set the envsphere's Strength to 0 and rendered.

The image is totally blown out by the primitive sphere's 500 Emission Strength. Just as expected.

Then I set the envsphere's Emission Strength to 1 and rendered again, leaving everything else the same.

So, the envsphere doesn't have to be contributing to the rendered scene; it just has to be emitting light, no matter if that light can be seen by the camera or not.