> Reversing here just means yielding back the inputs.

Not quite I think - the example gate they give has (a,b,c) as input and it doesn't return c. So it's not yielding all the inputs back.

Furthermore: if you always returned all the inputs, and also computed other values, the outputs from the gates would be strictly increasing in size, so you wouldn't be able to use a finite set of gates to build a computer of arbitrary size.