That is true. However, it also has a staggering amount of duplicate data. I have _heard_ that if you search for most any particular book, you often get a dozen results of varying sizes and quality. Even for the same filetype. It's a hard problem to solve, but if we had something that could somehow pick the "best" copy of a particular title, for every title in the library, Anna could likely drop the zero herself.
As one of their blog posts explains that's by design, they download all versions of any file. The reasoning was that some worse quality video files will have subtitles or better audio than the high quality video.
Some filtering may be possible to automate but lots of the tasks involved will have to be manual. Like merging video and audio from different sources or syncing subtitles from another file.
Honestly, if I can't have the whole thing, I'm not going to bother mirroring a 1TB fragment that's worthless by itself to everybody except copyright attorneys.
As ndriscoll points out, the only feasible way to distribute an archive of this size is with physical hard drives. I sure wish they would find a reasonably-trustworthy way to offer that.
Most of the books are bloated PDFs. I'm slowly working on a project to reliably convert PDF to DjVu, which on average yields a highly readable document that's 33% of the original size on disk. The project is proving difficult, as the tooling for DjVu is quite moldy now, and often needs to be manually reviewed to ensure the file remains readable. Pdf2djvu exists, but it's highly unreliable, and thus can't be used in bulk. Other ebook formats are XML-based and tend to be similarly bloated due to the overhead of the markup. It's a hard problem with so little in the way of good file format choices.
That sounds like a pretty terrible idea, TBH. All of the best tooling is for PDFs, as you note, and storage will only get cheaper.
Ultimately that content is going to need to be represented as raw UTF-8 text and encoded images, so I don't see much upside to migrating it from one intermediate lossy file format to another.
1 PB of disk space would cost about $10K at this point in time. Not exactly unattainable. Looks like it would fit in a volume of space about the size of a standard refrigerator.
22 TB drives are around $230 on ebay, so if you used 15 of them in raidz2, that'd be around $3500 (so maybe a little over $4k with the rest of the server), which is around the cost of a new mirrorless camera and a decent lens, so certainly within the realm of a hobbyist. You probably couldn't get away with downloading 250 TB in any reasonable timeframe with most US ISPs (or at least Comcast) though. That'd be over 2.5 months of 300 Mb/s non-stop. Even copying it from a friend using 2.5 Gbit/s Ethernet would take over a week.