But in reallife this is not possible, always there is wind.
Are you comfortable that you are not confusing drift and slip ? Chalk and cheese, I suggest .. Just how the box does it all is not in my ambit but it appears not to be a massively difficult exercise albeit that the real installations would be expected to use additional inputs to resolve any conflicts in the sums.
In the real world, however, a yaw vane/string makes it a doddle.
but reference is geographical position, in another word, earth, not airmass.
The accelerometers are looking at inertial references. The box then does whatever sums it is programmed to do to come up with some half plausible story as to what the aeroplane might, or might not be, doing. (Caveat - not my area of expertise so I can only look at things generically)