Citation most definitely needed. Perhaps the rationale is that the image is blurrier, or the contrast is worse, but both of those could easily be simulated by a monitor if you valued them.

I'd have taken it more as, reflected light would be better than straight to the eye light

How does the eye even "know" which kind of light it is? What's the difference, is one spectrum smoother and the other spiky or what? That's the part where a citation is needed.

Paper is more pleasant on the eyes for many reasons that don't apply to a reflective very bright projector screen.

This is me being a 100% armchair physicist, but I'd theorize it'd be something like trying to iluminate a room with a flashlight vs pointing it to a white (diffusive?) surface, counter intutively you achieve better* ilumination with the aditional diffusive surface

* Better for say, photography

Arguably what I think is actually happening is the focus distance has increased considerably.

The muscles that contract or relax to change your lens shape (to focus your vision at different lengths) don't work linearly. Most of the work/tension of the lens is happening in the last meters range. Such that looking out at a 10m distance or 200m distance is a lot closer in muscle tone then the tension applied to focus between 10m and 0.05m.

Most likely what is happening is that the visual field workspace is equivalent but the eye strain is considerably reduced from being able to not look at something so close by such as the displays we use every day.

The theory in the article is that this is due to reflected vs direct light but I think this is a post rationalization derived from the author's experience. The perceived experience is that it is easier on the eyes. The reason is less clear.