I don't think this is good advice. The risk is that the plane is in fact on time, and presumably you add in buffer time to make sure you make it if there is an on-time departure. You can't be half in and half out.

If the inbound is delayed (and it is already in the air so you know it is truly delayed and not fixable), then you are safe to arrive late (and add in your regular buffer time - if not a bit extra in case they can turn the plane in early)