It's not as if Horner tossed down a blanket in the shade and had a little rest... he stood on the side of the road likely about to pass out, guzzled a coke, smashed some food in his mouth, then got back on his bike and kept riding.
As far as the grupetto, I think back in the day officials gave them a little more leeway on the time cut. So, if a huge group finished way back but stayed all together they wouldn't kick them out of the race as long as they were "still trying", which resulted in a lot more shenanigans and relaxed riding at the back. Those days seem mostly over, as we've seen some big name sprinters sent home from grand tours after just barely missing time cuts. For the most part now it seems like those guys are working hard to keep the time gap in check and don't really mess around much.