You're assuming that it's in some kind of high-availability storage built for concurrency or I/O, rather than chucked onto old department shares slapped together in the early 2000s, or maybe numerous Outlook archives of departed employees that people can't bring themselves to delete... most of which is likely on tape or, as I alluded to, off-the-shelf HDDs, maybe even in ESD bags after being pulled from the ancient desktop they once were part of. Perhaps even a fancy SMB NAS was thrown into the mix a few years ago. And much of it is probably backups ort is replicated/superseded elsewhere, but no one has the time to figure it out.
That's the thing about the 100 TB figure... it doesn't take a lot of desktops and laptops that were turned in during because of departure, termination, or upgrade to reach it. But with various regulations about data retention, requirements to scrub other types of data before disposal, and just the inertia of government (just like in business), the better assumption is that this is spread across a hundred or more separate storage media, devices, and systems... and the consulting firm probably did the same, grunted out a number, then used a boilerplate conclusion with the subjects changed to match the industry.
You show me a company of more than 20 people that's been around for more than a decade dealing with data, even just emails and a website, and I'll show you where to find the TBs of non-operational or archived data that someone(s) can't let go.
Of course, it is possible that some of these departments do have it on modern storage solutions... but if that was the case, it's likely they aren't suffering from the same issues that the consultant identified in their summary. It's probably reasonably searchable, has sufficient redundancy, and may even have coherent archiving and deletion policies. Then again, I've yet to work for any company that can do this across all levels.