No, this isn’t correct — at 24h ahead, if any non-blocked, non-preferred seats are available, the seat map opens to allow selecting them. If such seats are available OLCI will automatically assign them as well. They can also be changed later, UNLESS you got an “assigned at gate”/seat request card (at which point only a GA can assign). Not sure why DL doesn’t make this more clear in their documentation, as it seems to be a very common question.
I think the confusion comes in how all 3 of the US3 have implimented different versions of BE. For instance United does not allow you to choose your seat at T-24, it assigns it randomly and you can't change it.