You only need 1 bit if you declare an encoding scheme where 1 == 1 million. You need 20 bits to define 1 million DIFFERENT luminances. Their ratio is subject to an arbitrary scaling.
> To express a ratio of 1000000:1, you need at least 20 bits.
You mean to express all the integer factors in the range 1000000:1, you need at least 20 bits. With 20 bits you can represent 1 times brighter, 2 times brighter ... 1000000 times brighter.
But there's nothing special about those coefficients. 20 bits does not allow you to represent 1.5 times brighter. That's still in the range 1000000:1 but 20 bits isn't enough to represent it.
If we're happy to skip 1.5 times brighter, why can't we skip all the even integer times brighter values, and use 19 bits?
Things get wonky if you don't have a linear scale with a true zero; in such a scale the low end of your N:1 contrast ratio (in the smallest representation) has a value of 1, and the high end has a value of N.
Also, if that type of "wonky" throws off your rendering pipeline, you're bound to get something else wrong.
Such as ever having a linear scale with a small number of bits in your pipeline. The linear scaled brightness stays afloat all the way through (cause floats have this handy feature of being transparently sorta-logarithmic in the way they use their bits, even 16-bit floats beat 20-bit ints for that purpose), only at the very end you apply the tonemap+gamma function(s), then dither, then truncate to fixed (8) bit integer.
edit: And aren’t output color spaces already highly nonlinear due to gamma correction?
I didn't interpret the articles premise that if you had 20 bits, you could 100% reproduce the light in a scene.