The argument is that since the light is the same, it can't have different effects. Reflected light cannot be inherently healthier than light from an LCD.

Also, if we go a little deeper, then intensity is really not intrinsic property of photons themselves. Photons radiate through space and are carriers of electromagnetic force. They have momentum but no mass. Its energy and momentum is a function of frequency (or wavelength). Furthermore, they also have spin angular momentum which is truly intrinsic as it does not depend on frequency.

So, the saying that light is just "intensity and frequency" is naive and flat-out incorrect.

Is it not sensible, though? People are trying to argue that light from a backlight is inherently more dangerous than reflected light. The easiest way to dispel this notion is to establish that the light from a backlight is just light like any other, which it is. At most, it varies in spectrum and intensity. Polarization is affected by filters outside of the light source. The phase that's hitting your retina varies entirely based on irrelevant things like when the light source was turned on and how far away it is.

Starting by pointing out that intensity and wavelength are the only intrinsic properties is a great starting place.

You still haven't pointed out any additional properties that have any bearing on how your retina is affected. I was wrong the say that there were no other properties, since we can endlessly discuss the quantum nature of light or how it experiences no flow of time since it travels at the speed of light or all sorts of other properties...

but none of these things change based on the light source. Light is light, except for the intensity and wavelength. Unless we're willing to consider things which could affect reflected light just as much as they could affect emitted light, since they're the same thing. Light.