99% 0f magnetic flux travels through/along laminations path because itīs the one with lowest magnetic resistance, thatīs the purpose of using a core, but still a little travels outside and can induce hum into other cores and/or any piece of wire, which can be considered a coil.
Even a short straight piece of wire *is* a coilbecause curent through it induces a field around and viceversa.
The external copper band shorts that outside flux.
Wonīt it short the transformer itself, overheat it, etc.?
Not really: coupling is very poor and to boot , lossy.