It is. From 5" Hg to just below 30" when the throttle is popped open. It is only negative relative to atmospheric pressure which is 29.92 at sea level on an average day. Engineers are now taught to think in terms of absolute pressure. There is no negative pressure in terms of absolute pressure. Your way of expressing this is not wrong but it is outdated. Using atmospheric pressure as a reference point can lead to errors in calculation and thought process of how this actually works...

BTW this also how it is taught to student pilots learning how to set a constant speed prop. The manifold pressure as shown on the manifold pressure gauge increases when the throttle is opened. You then set the prop pitch based on manifold pressure.