Did you know there are a lot of different types of iron? There are, and some are absorbed better than others.
After RNY, we don't have a lot of stomach acid. That's why we need calcium citrate instead of carbonate, by the way. You need lots of stomach acid to break down calcium carbonate so it can be absorbed but we just don't have enough.
You need a fair amount of stomach acid to absorb certain types of iron, including ferrous sulfate, ferrous gluctonate and ferrous fumerate. You'll absorb some of those, it's just hard to say how much.
You do not need much stomach acid at all to absorb carbonyl iron, though. So we absorb it better.
Also, ferrous sulfate and similar types of iron contain a lot of iron salts. That's why 325 mg ferrous sulfate only has 65 mg elemental iron. Carbonyl, on the other hand, is all elemental iron. Those iron salts cause a lot of constipation in many people. Carbonyl iron is much, much less likely to interfere with your ability to poop.
Now, there is also heme iron, which is a whole other thing. It's absorbed really well but it's expensive. I don't know much about it; Andrea is the person to ask if you wanna know more.