Re: ... what?
They are talking about effective capacity, which takes into account their deduplication (which they do on a 512B block size, so it's extremely effective), their compression (which is done on variable block size starting from 512B, so again, super effective), and their pattern removal. Thin provisioning has nothing to do with it.
Actual raw usable from the 5.5TB unit is about 4TB, because they use some space for their Purity OE and the Metadata tables and whatnot. The numbers they publish for RAW are just actual raw disk without any overhead applied to them, so the numbers are a little less. But their focus is on minimizing the data footprint rather than raw vs usable, they care about effective capacity.
It has everything to do with data efficiency. Their target markets are Database, VDI and Virtualized environments. VDI will give you 10x+ effective capacity because of so much duplication of data. Virtualized environments are up in the 6x range.
Database is not much for dedup but will compress ok. Dedup comes into play for database when people start making copies of the database, because Pure uses pointer based zero footprint copies that use just a bit of metadata. So Database can vary from 2x efficiency up to 8 to 10 or more depending on how many copies DBAs are making.
Now, you would never want to put unstructured data on these arrays that doesn't dedup or compress... that would be very bad and your $/GB would be through the roof. All Flash arrays aren't for every kind of data type until prices come way way down and densities go way way up.
Anyhoo--that's how they are doing it, and as someone who has installed a dozen of these things for various customers, it works fantastically well. Super fast with great efficiency rates, typically we see 4x-10x, DB on the low side, and VDI on the high side.