Maybe people around you use to say these kind of things. But it is not commonly acknowledged. I often wear heavy backpacks and I never had back problems. I cycled for years every Saturday from the cheaper supermarket to home more than 5 km with a backpack that with all the groceries weighs more than 10 Kg. Actually the Saturday ride is the less problematic because when I get home I can change, sweating on the back is a common problem and it may be as you say "acknowledged".
Wearing the backpack on the chest would probably worsen the way the weight is distributed*. It maybe useful to shift it for some minutes during the commute to let your back cool down before you start sweating. But if you have some back pain for some other reasons riding in this way you would probably be worse off.
If instead of 5/10 Km commutes you are talking about longer rides wearing a backpack on your back or on your chest won't change the result. it would cause a lot of sweating anyway.
*By worse weight distribution I mean that while the backpack is on the back if you are slightly leaning forward the backpack is leaning over all your back and the weight is distributed. If the backpack is on your chest all the weight is on the straps which then pull your shoulders.