Indeed, brushing one s teeth with a toothbrush or a miswaq is strongly discouraged at any time from noon onward on the day of fasting. It is needless to say that using toothpaste is not allowed, because of the likelihood that one may swallow some of it or of the water with which he rinses his mouth. But even without toothpaste, the use of toothbrush is discouraged because it changes the mouth smell. That very smell is preferable to Allah to the smell of musk. Musk gives one of the best smells known to man.
All people like it especially because it overpowers any bad smell in the vicinity.
When musk is considered inferior to the smell of the mouth of a fasting person, the honour Allah gives to those who fast appears to be great indeed. We may note here that this statement is given in a figurative way. Allah is not affected by smells, foul or pleasant. He does not need a good smell in His presence, either to expel any bad smell around or for its own sake. What the statement means is that Allah rates the mouth smell of a fasting person higher than we rate the smell of musk.