Since the output sequence of a 1b-quantizer only contains 1s or -1s, it's output power is constant. As the input power can change, the gain
of the quantizer changes correspondingly.

The loop gain of one is not forced. But it is rather a condition for stability. Assuming the input power of the quantizer increases, the 1b-quantizer
gain drops and the modulator will become unstable.

Regards