No it's correct, some of the Silicon Image SATA Raid chips support RAID 5. The one on the Asus A8N-SLI Deluxe / Premium also supports RAID 5. I'd guess it's the same chip.
However, the main processor will do the XORing to generate the parity stripe and you can't boot from the RAID 5 array. Hence it's not that useful.
To use RAID 5 realistically you need a RAID card with an onboard processor and cache and preferably, PCI-X or PCI-Express.