Nope -- you didn't check the link he provided. The FillTrol is expansion tank and pressure-regulating fill valve in one unit. The air pressure in the tank bladder is what regulates the system pressure.
Phooey. He HAS to drain the FillTrol tank, at a minimum -- and since his problem is that the system isn't full, he can't possibly avoid admitting fresh water to the system. He doesn't have enough water in it. There is _no_way_ to fix his problem without admitting fresh water.
In any event, as soon as the oxygen that's in the fresh water is used up, there won't be any *additional* corrosion. The real big corrosion problem in hydronic systems comes from the *continual* admission of fresh water in the case of a slow leak (which is why it's recommended to close the fill valve once the system's full).