I think that was the opposite of iDave's plan, as least as he originally described it - he didn't present it as a long term dietary change but as tool for shedding weight as preparation, for instance, for a sporting event. Thats not to say it was necessarily faddy it was just a programme with a short term goal.
You are right as far as I'm concerned: this is the title of the document I have (relevant part bolded for emphasis!)
RST Low GI fat loss programme (AKA the iDave diet)
If you think about how the science (allegedly) works then it makes sense to be a fat loss program, as by controlling insulin production you encourage your body to use stored calories for energy rather than keep stockpiling more.
So yes, if you have always eaten healthily & moderately and have never been overweight then of course you can carry on as normal, but if like me you let yourself go then the iDiet is great for getting back on track.
FWIW I still try to eat this way as much as possible even after reaching my target weight but am less strict now.
If somebody ate 5000 calories a day of high fat low carb food for three weeks, would they get fat?
As has been stated, it entirely depends on how many calories they expend. Also, low-carb does not mean no-carb.