The LEDs are hooked up using i2c to a driver chip – a is31fl319x which has a corresponding userspace driver leds_is31fl319x
The is31fl319x is located on i2c bus 1 at 0x64, and on ArchLinux, the driver module leds_is31fl319x loads at boot. If you run i2cdetect -y 1 you see that 0x64 has the value UU, indicating that the driver is controlling that address. lsmod confirms this.
If you want to control the LEDs you can write code to talk to the driver.
Alternately you can remove the driver (rmmod leds_is31fl319x), which will allow access to 0x64, and you can use i2c (i2ctransfer) to send instructions to the is31fl319x directly.