You are correct. The discounted bands are available to guests who have booked a resort and ticket package with Disney.
A MB is nice to have but not necessary. If you prefer not to purchase MBs you will be issued a card the first time you enter a park and use that for access to LL lines and the parks. We were at
Disneyland last summer and had cards snd they were very easy to use.
Our family won’t be purchasing MB+ until they come out with a version that links more easily and has better battery life. We will be using our old bands until then.