Though I understand your remark (one red wire for + and and blue wire for - from the transformer makes sense), the wiring diagram is correct and I can assure you that it’s working fine for me 
The wiring in my house from the transformer to the button and chime was originally with 2 red wires connected to the transformer. Seems to be a normal solution, see the diagram below.
Guess it’s just a matter of taste how to wire it so the circuit get’s closed when pressing the button.
