A global variable of a subrange type is also initialised with 0, even though that is not a valid value for such a variable.
In the general sense, it is impossible to define Default in a way that always results in a valid value for all variables, because it can also be used for variant records and then you can have overlapping fields of different types and an impossible situation to pick a value that's in the range of all of these types.
So you cannot rely on Default assigning a value that's in range. Since initialising is not the same as assigning a value that's in range, the documentation is correct.