That’s because they want you to use the Binary_crossentropy loss function with the from_logits = True argument, which causes the sigmoid calculation to be incorporated in the loss computation. That is preferred because they can manage numerical stability better when they do the two together. E.g. dealing with problems with “saturated” sigmoid output values. Here’s the doc page for Binary_crossentropy.
They don’t really explain what the from_logits = True argument does in the assignment, but they do literally write out the code for you in the instructions. Maybe it would help if they explained it a bit more. I’ll suggest that.