Its not the RAMDAC. RAMDAC = Random Access Memory Digital to Analog Converter. There is no Digital to Analog conversion going on when you sending a signal to DVI/HDMI/DP monitor.
What I guess might be happening is that its using the same clock for the TMDS chips. Every single graphics card over the past decade has had a 400 MHz RAMDAC (for the DB-15 VGA output) which is not used for the digital output.