I drink two cups a day, and I've read that too. It's the tannic acid, or tannins in the tea. It's also found in coffee though. Between tannins and caffeine, it does reduce iron absorption. But I'm not entirely sure if it would make that huge of impact to a person; but our sources of iron being non-heme, it's probably worth looking into.
Personally, I think there are so many amazing benefits to drinking green tea that it would be worth the risk, if it being that big of one. I just make sure I get plenty of legumes and such, as greens like spinach is not a good source because of the oxalates it contains. And plenty of vitamin C to increase absorption!