The reason I'm going for firmware is while the HMIs could have had a solarwinds style exposure, but that's just any generically wormable OS vulnerability, and not something that should cause a physical shutdown.
To shutdown a pipeline, it's not a management console issue, hence why I'd speculate it's in the ICS devices themselves, which probably use uClinux toolchains on SoCs from one or two large vendors. I did some smart meter and ICS security work in the 00's, and there were a few vendors who would be strategic targets. The attack tools available now are unbelievably better, while the attack surface is pretty much the same due to the long lifecycles of ICS components, and considering today we've got cheap SDRs and gnuradio blocks for most wireless protocols, AVR tools, buspirate and the good/greatfet, ghidra/ida, and python for reverse engineering, the vulnerability research on this stuff moves way faster than the industry ability to respond.
If this is a serious attack, the only way to respond will be if they are very lucky, it's a worm and they can stand up a honeynet with spare gear to catch a sample and any good infosec firm can pull it apart. But if it's an active APT group, there's probably a political solution, as given what's possible, this would seem to be just a shot over the bow.