If you put it in Glacier Deep Archive it’s closer to $1.3M per year, just to set a lower bound.

Deep Archive has retrieval times up to 12 hours, so for on-prem/colo that is then competing with buying a few tape robots (you can get a single tape archive that handles >1 exabyte, but you'll of course want redundancy without having to have staff shuffling tape in an archive that will run into the thousands). You can buy management software that tries to replicate a glacier-like experience for things like SpectraLogic tape libraries but they may not be the cheapest option.

It ends up becoming a very different beast than the storage classes w/fast enough retrieval guarantees to need disk, in any case.

Unlikely that they want it hot or even warm. That data almost definitely went to Gkacier while they worked out what to do with it.

You overestimate the enterprise.