Actually, no, it is not illegal PERIOD. The law mentions cases in which it is required.
For example, a school bus or truck using inflammable liquids is required to signal it is going to stop at all railroad crossings and to continue until having passed through the crossing.
You can not both do it and not do it at the same time.

As to signaling intention to turn and using four way flashers, they use the same lights so all you would see was the four way flashers. Since the four ways are required and do not allow turn signals simultaneously, the four way telling everyone nearing it that there is a hazardous situation is signaling a possible turn, possible breakdown, possible about to explode or any other hazard.

Drivers approaching such situations are required to use their brains and judgment. (We all use poor judgment from time to time but that doesn’t mean we should pretend to justify it later; hat just means we intend to do it again and haven’t learned a thing from the experience.)

The idea that every driver thought he was broke down and so they were passing him on the right would mean they weren’t looking at what he was actually doing. Since several watched the one who hit pass him to go run into the truck, it appears they did NOT assume the truck was stopped and that passing on the right would be hunky dory.

Note the driver of the tanker was not ticketed by the officer who (we assume) knows the law forward and backward for doing what he was required by law to do.

David for self-driving cars ASAP

29 is a US highway, not interstate, & if you hit someone from behind when they slow or stop to make a turn on a US highway, you are automatically in the wrong. When you see a big truck ahead of you going very slow or stopped, find out what’s happening, don’t just zing around it like you own the road. When you pass a truck or are cruising along next to one, you are in danger, get on by or drop back, just my philosophy.