You are right that fortified foods are the same as taking a supplement. But I guess for those that can't have dairy, it kills two birds with one stone. I don't consume diary but don't use fortified products as I'd rather take a good quality supplement if I need to. Tums contains calcium carbonate which is not as absorbable as calcium citrate, malate or MCHC. Calcium carbonate is the least expensive option so that may be a consideration and it must be taken with food to aid in absorption where the others can be taken on their own. Cabonate is also the most constipating form if that is an issue for some so lots of things to consider in where we get our calcium from.
An interesting tidbit...eggshells are almost completely calcium carbonate and are edible if you cook them. After cooking (lots of info online about how to do this and it's really easy!), you grind them into a powder and add to food. A small amount packs a really big punch of calcium so if consuming, you should only take a tiny amount at a time, certainly not the whole shell at once as it would be too much. Easiest way is consume is in applesauce or soup. I'm not sure if the carbonate in eggshells is as constipating as it is from supplement sources but I would think not. It is likely more absorbable than a carbonate supplement as it has other supportive nutrients vs. a calcium carbonate supplement which is generally produced from rock. There are now supplements on the market made from eggshell but if you eat eggs at home, it's a simple process to make your own and then your calcium supp is free! A friend of mine had chickens and they were laying eggs that had soft breakable shells. She then took those shells, cooked and ground them and added them into their food and pretty soon, they were producing eggs with nice sturdy shells as they were getting the calcium they needed.
I know, I know, this is likely too much information but I get excited by the various options that are out there!