I have to agree with Professor Belkin on this one. Great orchestrations balance themselves. Having conducted hundreds of scores I can tell you that from a conductors standpoint seeing such a thing puts up all sorts of warning flags. Its not always wrong and there are times when it is very useful but many young orchestrators do not have the experience to handle such a special situation properly.
Another point that Professor Belkin has made in this course which I hope he will elaborate upon is the concept of Foreground, Middleground and Background as well as rhythmic variance and register segregation and overlap. With a good understanding of these principles, you can begin to determine a very aesthetic manner to balance that low flute
A great example that Im sure you have seen is from the Forsyth orchestration book which is your statement almost verbatim (Pg 185 2nd edition). In Forsyths example, that wondeful little country tune from Dvoraks New World Symphony a solo p flute against ppp strings no bass works but if you look carefully, the flute is segregated in its own register, is an enitrely different rhythm, and is a foreground statement that is so obvious to the ear that the ear is immediately drawn to it. To tell someone that their orchestration would be successful simply because the strings are 2 dynamics lower than the flutes is cautionary at best.