The request for pure salt is not necessarily only about iodine. There are salts that contain anti-caking agents and other things that might be bad. When you are making an extraction and mixing different chemicals, its better always to avoid adding unknown variables but rather have pure products to have clean and expected products.
So even though it seems to me (but as I said, better wait for second opinions) that otherwise-pure iodized salt is no problem at all, still, if you can get pure sea/rock salt (sold in any of these more 'natural' stores or even in some supermarkets), its always safer/more desirable