Still doesn't add up. Each 3U can hold 10 x modules = 183TB raw. But the 1PB effective by your maths would need 20 modules. Either the register or Pure are being a bit misleading. The 1PB effective, based on 5:1 would be in 6U, not 3U.
In fact reading back they are definitely claiming 1PB effective in 3U (ie. 10 modules) so they ARE claiming 10:1 which is very 'optimistic'.
"1PB effective capacity in 3U, 183 TB raw!" ... Hmmmm.