It’s actually based on observation: many noticed that they needed to add x EV exposure to get “neutral gray” at the proper spot, compared to what the camera said. So hardly an assumption. And that observation dates from the introduction of filmic. You won’t notice this with the standard basecurves, as they also apply that additional exposure correction, but it is hidden in the curve.
And of course, you can always compare what your raws show relative to your in-camera jpeg (under default camera settings). Or compare what the basecurve module gives compared to filmic.
Oh, and in case you are talking about the automatic compensation of in-camera exposure correction, there’s another thread where this has been treated ad nauseam