So after talking to my colleagues I can verify that 15°/min is correct however the sentence was, a bit short frankly and the explanation lacking a bit. What the old sentence meant was that the automatic synchronization would cut off at above 15°/min. After returning to a straight and level flight the automatic synchronization would kick back in. That takes about 20 seconds to do. If it should fail you can manually synchronize it with the mode selector knob on the compass control panel. It is now explained with a bit more detail thanks to everyone that brought that up