Think when they say available in December to existing customers they mean available to order rather than available for guaranteed installation.
The reason for the lack of slots for installation is probably a combination of factors.
Limiting the amount installed initially to stagger the rollout nationwide... so in the event there are any problems it can be halted easier.
the box is technician only installation, no self install.
the box is allocated a longer amount of installation time than current equipment to allow for connecting the box to the broadband modem, moving the broadband modem closer to the set top box if required, technical issues that arise during installation, or even just the technician being unfamiliar with the new technology beyond the basic training provided.
As a result they cannot install as many per day, week or month as they still need to do their other installations also.