I read on another site (sorry, don't remember it now) about juice fasting and I think their explanation is more accurate from a biological standpoint.
They basically said that when you are in your fast you are basically starving your body. So with nothing new coming in your body starts to digest whatever it has on hand. This is both good and bad. Good in the sense that all that old scar tissue and weak or compromised cells will be broken down for raw materials for new growth. But bad in that some perfectly good muscle tissue will also be cannibalized for much needed amino acids.
The site said that is why they prefer juice fasting (they suggested a number of juices that were good) because it provides a minimal amount of carbs, fibre, etc to meet some basic needs.
So the "biologist" is scientifically right. However, while your body isn't in a "detox" mode, the end result is that it will remove a lot of unwanted material. So you COULD call it a detox mode. Think of it, if you stop supplying your home with fuel, you will start to freeze, so you will start burning garbage first. Eventually, the garbage runs out and you start burning books. The process continues until you are burning furniture and tearing the walls apart for spare wood.
Apparently the body works in much the same way, preferring to scrap the useless, diseased and unwanted parts first. Of course, fast long enough and you start to cause harm because you starting buring your furniture and structure. :)