I use a lot the HOLD function too. I prefer it to leaving the foot on the brake at a red signal stop.I don't time it but suspect that after fiddling with seat belts and such, at least 30 seconds has transpired before I push off. WRT the clutch on the PDK it is my perception that as long as the brake pedal is firmly depressed, the clutch remains disengaged. This is certainly the case while at an intersection stop. I intentionally frequently use and appreciate the brake HOLD function at stop signals for that very reason - full dis-engagement of the clutch until ready to proceed when clutch engagement and brake release is initiated by application of the throttle. This is IMO a very well executed function by Porsche engineers.
But, it don't think it disengages the clutch, because as soon as you push the gas it moves.
If I've to stay a lot instead, I put into neutral pushing both PDK paddles.