I've been driving for a number of years. I've seen people doing their makeup, eating, trimming their fingernails, shaving, and reading (all before self driving cars were a possibility). And of course, drunk, drugged, driving while exhausted, or texting are all a possibility.

It is unlikely that the truck driver drove into the cyclists without at least knowing that they were there, but there is a possibility that he was just unfit to operate a motor vehicle.

Edit: just to add ... as a cyclist, I've been pushed off the road by drivers, and I've had a tanker truck blow their horn at me while passing mere inches from my handlebars at 60mph ... fuck cars, but I'd like to hope that most drivers aren't out to kill us for not driving.

People absolutely do that. A few weeks ago I was almost taken off my bike when someone ran a red light because she leaned all the way over into the passenger footwell space to retrieve her phone from her purse while she was still moving towards the intersection. She was still going ~10mph and took her foot off the brake entirely and put her head below the dash. People take their eyes off the road and hands off the wheel for like ~10 sec to fiddle with their phones, which is a lifetime while traveling at 100km/h.