I think the reason for that labeling has more to do with the lack of dissolved ions than bacteria. There shouldn't be any bacteria in that water. But just so you know, bacteria are killed at 150ish degrees. No boiling required.
DI water is pretty pure water and can be corrosive to metals. In principle, then, it should also be corrosive to internal body tissues (hence the safety warning). But, of course, normal humans are constantly ingesting replacement ions in their food and beverages, so I don't think there is any safety concern. Malt is loaded with ions and so once you add malt to DI water, it becomes simply water.