Maybe you are confusing AC sinusoidal with BLDC?
Also AMC maybe thought you were referring to AC sinusoidal command signal to the drive, where the controller provides two phases and the third is interpolated from these two.
If halls are used in an AC sinusoidal commutation it is just to locate the initial position of the rotor, after that the encoder is used to commutate the 3 phase, or a resolver can be used also.
This link shows the BLDC trapezoidal drive.
A study of the different types of AMC AC sinusoidal drives will explain the different methods used.
Some accept 10v analogue, others AC sinusoidal signal.