It's not.
a) compression can be lossless.
b) RAW is not about storing literal photons ADC measurements. It always has "some" processing as those always go through an ISP. We can obviously discuss which processing is the cutoff point and it will differ for different applications, but typically this would include things like clipping, sharpening, or denoising. And even some pro DSLRs would remove row noise or artifacts in supposedly "RAW" files!
If you can change the exposure or WB - it is what is the minimum practical/useful definition of a RAW.
If you can change the exposure or WB - it is what is the minimum practical/useful definition of a RAW.