So yea, your instinct is correct. You can use ARHD 12 with your current firmware, but it supposedly works better with updated firmware that HTC is starting to roll out OTA. Said firmware is NOT built-in; it needs to be flashed separately. After confirming a few times in the ARHD thread, I basically use Revone to S-OFF my phone, then give it a new SuperCID, and then flashed the new firmware.
I got S-OFF by following the instructions in this
post. Revone succeeded on the first try, so I only rebooted a couple of times when requested. No "reboot and try again" like it's implied.
I then flashed the firmware using the instructions from this
post. BUT, I did NOT use the firmware zip he linked to. I used
THIS FIRMWARE instead; Electronic Punk removed recovery & boot, and he also added the SuperCID of 11111111. I had a mild panic when, during the "fastboot flash zip" part in the OEM Mode (where it's a black screen with the HTC logo), it said it couldn't do something, but I followed the instructions and told it to "fastboot flash zip" again, and it went through.
Now I'm using ARHD 12 with the latest HTC firmware on an S-OFF phone. Yeesh!