That is Video RAM for the GPU, though not the main RAM for the system. The minimum RAM amount you can buy the iMac Pro with is 32 GB, 64 GB and 128 GB configs are also available
Also, 18 GB was certainly a typo, if you look now, they are saying 16 GB now.
Also, 18 GB was certainly a typo, if you look now, they are saying 16 GB now.