Every single other car ever made with a range estimation functionality uses past data to estimate your range. For like a decade Toyota had this on nearly every vehicle despite none of them having GPS integration, and nearly all of them were accurate enough to rely on.
You are bending over backwards to come up with an excuse for Tesla, when they quite clearly made a choice to either be lazy or misleading.
When my gasoline car says "You have 12 miles left", I KNOW I can drive 12 miles minimum, as long as it isn't at wide open throttle. The car has mountains of data at it's disposal, but in reality this technique has been used ever since a car could figure out how much gas was flowing into the injectors. The assumption is that how you have driven for the past ten minutes is a reliable indicator for how you will drive over the next ten minutes, and for most ten minute periods that is an accurate assumption.
