Objects that block a certain amount of light, and then emit a certain amount of light: You can only make that simplification if the entire world is evenly lit by white light.
Blur the entire matchstick into one pixel. Under white light it has to emit brown plus yellow. In a dark room it has to emit just yellow.
It's a clever technique but I can't figure out any way it's not fundamentally incompatible with having lighting. If one pixel has to both be lit and emit extra light, you need two RGB values.
RGBA in associated is unique in that it represents distilled geometry. If you were to zoom in on a pixel that had a degree of geometry occluding it, the coverage is represented by the alpha ratio, while the RGB is purely emission.
It is complimentary to lighting.
The math works because of the differing forms of the alpha over formula. FG.RGB + ((1.0 - FG.Alpha) * BG.RGB), resulting in a pure add at the extreme case of alpha being zero, or ratios of addition when non-zero.
Within the limitations of the RGB model, it works extremely well.
The folks cited are extremely adept imaging people, covering years of experience and several Academy Achievement Awards.
They're plenty smart but they're talking about a different use case.
You don't know what color will be emitted unless you know what light is hitting the surface.
A dark glowing surface and a bright non-glowing surface have the same emissions under white light, but different emissions under other kinds of light. The method you're talking about requires the emissions be precalculated, which means you can't apply lighting at runtime.
I agree with you, to do it properly you need two different RGBA objects, the emissive one will have A=0. But the same format suffices to represent both.
Anyways, read the links I provided above.