Activated Carbon (AC) does remove chlorine and chloramine from water. That comes from my former professor at University of Florida who specializes in activated carbon treatment and research. He did his PhD at Penn State where the top experts in activated carbon chemistry are. Any source that tells you that AC cannot remove chloramines is incorrect.
Charcoal is not necessarily AC. You can convert a carbonaceous material into nearly pure carbon in an oxygen-less retort oven, but its not AC until you have treated it in a specific way. While the carbon is very hot, injecting oxidants like steam, carbon monoxide, carbon dioxide, and hydrogen is how AC is created. Air or oxygen are not used as an oxidant since that acts too quick and destroys the carbon matrix. The result of the activation is that teeny 'pocks' are etched throughout the entire carbon matrix, creating a huge surface area. This occurs on both the exterior of the particles and deep inside the particles.
As pointed out in some posts above, chloramine removal is more difficult than chlorine removal. For that reason, the residence time for the water to be in contact with the AC is greater. For many contaminants, AC actually adsorbs the contaminant to the carbon. That is not the case with chlorine and chloramine. For them, there is a chemical reaction between the AC and the chlorine compound that destroys the compound and consumes the carbon.
Campden tablets are very effective in removing chlorine and chloramine. This is a well understood chemical reaction that is frequently used in wastewater treatment. As mentioned, dosing at a rate of about 1 tablet per 20 gallons will result in consuming up to 3 ppm chlorine or chloramine. That level of chlorine residual is typically on the high end of disinfectant in water distribution, but sometimes utilities have to superchlorinate the system for various reasons. The reaction produces sulfate and chloride at very low ppm levels. You can read more about this on the Water Knowledge page of the Bru'n Water website.
I suggest that a brewer would want to avoid overdosing their water with too much Campden tablet since that will result in an excess of sulfite in the water. You may know that wine makers add far more Campden to their grapes to kill wild yeasts. That does result in elevated sulfites in their wine and requirements for warnings to their drinkers. Some have mentioned that sulfites are boiled out of the wort, but I'm not sure of that. A quick web search does not produce any confirmation that sulfites are boiled out or converted. For that reason, I would try to match the Campden dose to the amount of chlorine compound in the water and the volume of water to treat.
Do be careful if your utility uses chloramine in the water. Many utilities change over to chlorine disinfection for a short period (typically early spring) to improve the disinfection in their pipes. Chlorine is a much more effective disinfection agent than chloramine, so more 'bugs' get killed in the system this way. One side effect is that the water smells much more 'chloriney' during this period. Part of the reason is that chlorine is more volatile than chloramine and it goes into the air much easier where you can smell it.
Having a simple swimming pool or aquarium test kit on hand to check the level of chlorine compound in your water is a good way to avoid overdosing with sulfites.