Don't know where you read that. If nothing gets to your stomach but water, what's to turn on?
I have done many fasts, and always brush my teeth, and tongue. Nothing is every swallowed.
When your tongue is clear of the white gunk, the bad breath should go away.
Joe, do whatever you think is best for you. If there's no food what is there to digest?
I can't argue the point because I don't understand the problem.
For me in 45 years, or so fasting I always brushed my teeth, and scaped whatever gunk off of my tongue that would come off.