Does all fire only emit light on its outermost shell?
The answer is NO.
The article you quote makes a completely unwarranted generalisation that &quot;all flames are hollow&quot;. This is true of some flames but only because the fuel that is burning is only able to burn when mixed with oxygen from the air. In those flames, the flame is &quot;hollow&quot; because only in regions where air can mix with the fuel can the fuel burn and the rate of mixing is dependent on the flow of fuel and the rate at which air can mix or diffuse into the fuel rich region. Bunsen burner flames are like this when the air is not premixed with the fuel gas (because the air intake on the bae is closed) and candle flames are like this because the wax has to evaporate to mix with air.
A bunsen flame when the air intake is open is not like that as the fuel gas and air are pre-mixed and can burn throughout most of the region of the flame.
Another factor worth considering is what causes the emission of light. In candles and many other poorly controlled flames, there is a lot of light from incandescence not from the underlying burning reaction. Incandescence involves black-body emissions from hot solid particles. Those particles are usually small particles of solid carbon produced from partial, incomplete burning of hydrocarbons. They emit light because they are hot. Candles are tuned to create a lot of these particles as this gives far more light than complete combustion. In contrast, a well-controlled bunsen flame emits very little light (the light is not black-body emission but comes from specific excited states in the reactants).
As to how this applies to your specific examples here are some analyses.
1. Campfires
Campfires are a mixture of various phenomena. They have flames  a little like candle flames. But they also have very hot solids which emit a lot of black-body radiation because they are hot. Some of the emitted gasses have little emission of light as they are like a bunsen flame, but the overall effect is dominated by the hot solids. And, of course, some camp fires have far better mixing of air and fuel than others so it is hard to generalise.
2. Rocket exhaust
Most rockets need to be as efficient as possible. So they tend to have very carefully designed mixing of fuel and oxidiser to achieve maximal energy output. They are not hollow and tend not to emit huge amounts of light (compared to the equivalent candle flame anyway).
3. Oil platform explosion
Impossible to generalise. But unintended explosions don't tend to be well controlled so will have some areas where mixing is good and others where it is not so some areas are like a candle and others like a bunsen. Incidentally, movie explosions emit a lot more light than real explosions because, well, hollywood likes visual spectacle not realism (see this youtube explanation and illustration of how movies deliberately add fuel to create poorly mixed candle like flames for more visually interesting explosions).
4. The sun
The sun does not emit light because of burning (at least not in the normal chemical sense). The sun emits light because the surface is very hot. And this happens because of nuclear fusion deep in its core (not what a chemist would call burning, though a nuclear physicist might, but what do they know?). The light we see is mostly black body emissions because the surface is hot. It is certainly not hollow either.