OK so we're talking about 2 different things...
A belly band, in the post-natal sense, exists to help close a diastasis recti. Since a lot of exercise can actually exacerbate an existing diastasis, the band assists in holding the rectus abdominus together, in theory, re-training the muscle into the proper position.
The "lose your mummy tummy" book works on your transverse, which is the muscle underneath the rectus abdominus (6 pack muscles down the front). Once the transverse is properly strengthened, the overlying muscles can close properly when trained properly.
That said, it sounds like your MT is concerned about uterine prolapse, which is DRAMATICALLY different. I would suggest you talk to your OB about that, if that is genuinely the case. That is not something that a massage therapist should be "treating" you for or making recommendations around. Not to mention....how would he/she even know that that was genuinely something you are at risk for? I'm assuming they were not present during your deliveries, nor your post natal internal follow-ups....
Either way, if uterine prolapse is seriously the concern, a belly band is not going to help it.
Go see your OB.