They explicitly said 24 racks.
"standard 48u racks"? NTAP has never sold 48U racks, they've been 42U for a long time and it was not uncommon for clients to install to their own racks (which were sometimes 38U). 1008U / 42U = 24 racks, so they were probably NTAP racks.
The MK14 shelves were 3Ux14d, which were generally replaced by 4Ux24d shelves. They could have had both. So, somewhere between 219-279U were used for disk drives. The larger controllers were often 6U ea (12U/pair), midrange were 3U ea (6U/pair). We don't know how many controllers they had, but if it were a 24node C-mode setup (which I doubt)), they could have had 144U for just the controllers.
That leaves +400U for switches and deadspace. So Pure replaced a lot of half-empty NTAP racks. There was probably a lot of related IT transformation/refresh activity going on in concert with the Pure deployment, this was not a simple competitive swap-out.