Okay - it wasn't HHO, it was honest plasma spoken of in the video.
"Thermal inertia" doesn't float my boat but what he may have not covered properly is the temperatures of the fuel. He was saying that the fuel gets hotter, then colder, then hotter than the exhaust. That actually is explained by the venturi. By increasing the speed of the gas (vapor, not fuel) through a restriction, the pressure drops as well as the temperature. The lower temperature means there' more heat transfer into the low pressure gas in the venturi. When the gas (or plasma?) slows back down after the restriction, the pressure builds back up and the temperature increases but has the added heat from that hot-to-cold transfer in the tube. It's not "thermal inertia" but standard thermodynamics.
I don't buy the hot versus cold separated by a tube generates electric fields. Lightning apparently requires water, ice, and snow.
The speed of sound could be interesting for supersonic flow transitioning back to subsonic where the transition between speeds is effectively a step function. Ions are just a gas. A disassociated gas with charged ions, but still should hold up to normal thermodynamics.
So. If one can convert the incoming fuel into a flow of plasma, what good does that do us? When the plasma mixes with the air, the reaction should be more violent than with the flamefront because combusting fuel needs energy added to it to disassociate into ions in the first place, energy that has to come from the reaction (or the spark ignition). Sulfer won't be eliminated. I'm wondering what would keep CO and NOx formations from being a normal part of the cycle.