Gonna ballpark in stupid units to see how wrong my intuition that that's not very dense is:
Assuming dense but not hot new thing spinning rust, 16TB per 34.5 cubic inch standard 3.5" disc.
(100PB/16TB)*2 (assume at least two spindle redundant) is about 12,000 discs, so about 414,000 cubic inches of just discs without any of the supporting equipment.
A highwall shipping container is like 5,900,000 cubic inches, so only like 7% of that thing would be discs.
Or, accommodating a little bit of the support, let's say it's just full of those commodity 90-bay 4U storage servers. Those are 19" x 7" x 26.4" (3511.2 cubic inches) for 1440 raw TB each, again 2 spindle redundancy so you'd need about 140 of the things for 100PB, round up to 500,000 cubic inches of those... still less than 9%.
Yeah, unless I did my math radically wrong, that's surprisingly not very dense.
Yeah, it really sounds a bit low. I'm not sure what else goes into these containers. I assume there might also be a bunch of portable equipment and cabling that goes into moving the data in and out of the container? Power infrastructure and cooling and what probably takes up quite a bit of space as well.
We have 1 TB microSDs. A pigeon could probably carry at least 100. If I did my math right(which i probably didn't), if it takes 24 hours of travel that's still a 8Gb/s connection.