Half true. If you feed a rabbit only lettuce, you're going to run into problems. Lettuce was bred for human consumption, low fiber, and not much else in it anyway. Rabbit digestion has other needs than ours.
Iceberg has a bad reputation because it's virtually empty, just structured water, no nutrational value to speak of.
As addition to a healthy diet lettuce is perfectly ok, and they like it. As staple food, no, it's just not enough.