The Tenacious D fucker already took back his apology anyway. Grummz is both right and wrong here. His main point was:

The majority of people don't actually have their own opinions (or rather, convictions)...

Those are the ones he wants "forgiven". But for the people intentionally pushing the Overton window:

These people are unredeemable and we should never forgive them for what they did to our country and much of the world, for they truly are destructive and violent and not really part of the social pact.

What he seems to miss is, as you say, there has been no "moving" of the window at all. What happened was some of the big influencers didn't want to look tacky and have people attack them, or make Trump look more like a human, so they took a few days off and dialed back after the shooting. (in some cases went completely silent like Hillary) This stopped the propaganda machine that NPC normies live under for just a moment, long enough that some of them started acting like normal human beings who don't wish death on someone. When enough time has passed, the hate machine will dial back up and they'll be back to parroting their friends on how much Trump is a threat to Democracy. Their opinions and "come to Jesus" moments are fleeting and irrelevant. So no, there is no need for forgiveness.

Only people with agency need forgiveness. What's the point of forgiving stupid NPCs? You tell them to get in line and force them if they won't comply. Troll them to oblivion. Out of compassion or for optics, we might want to consider forgiving the influencers and powerful voices with agency who actually changed their tune and genuinely continue to do the right thing.