The power requirements would be precisely why a Glacier HDD rack has only a fraction of its HDDs powered on at any given time. This also explains the 3-5 hours latency: you have a queue of jobs and you have to wait for other jobs to finish (eg. reading gigabytes of data) before your drive can be powered on.
That density is doable if the drives spend most of their time powered down, which would fit with the potential Glacier restore delay while they wait for the discs you need to come round in the power cycle.
