That shape allows them to fire artillery from multiple directions and to make so that any unit will be under fire whenever it moves.
In did a bit of googling and I learned two things.
Firstly, the Geneva Convention only applies to irritants or chemicals which prohibit breathing directly. It doesn't seem to include oxygen deprivation - taking
away a chemical. Gas exchange in the lungs begins to fail at around 19%, and you're unconscious as low as 16%. The exchange of gases in the atmosphere isn't exactly
fast - humans are less vulnerable than vehicles which will fail after only a few seconds, while humans take a little longer.
Secondly, thermobaric doesn't necessarily mean complex. Literally "anything which burns really quickly and can be stuffed into a shell" - It uses air as part of the reaction, and people generally need that. Even without the explosive effect, it's
still dangerous to anyone within an area. It creates a little "dead zone" of depleted oxygen and increased combustion products which lingers a little bit after the explosion.
Obviously there's complexities like wind and shit, but unlike any other munitions, every thermobaric shell just
makes the dead zone bigger. Each detonation just displaces more and more oxygen, far more quickly than any other explosive. A thermobaric barrage would just choke out everyone who doesn't get killed by the explosions themselves.
It's similar to the effect the Chinese have. There's a huge chunk of ocean around China where the fish can't survive. It's not that hard to do the same to people. That unit wouldn't have to move out of cover to feel the effects.
Any type of sustained barrage which uses air in its reaction would create such a dead zone.
Edit: to add, I forgot it would be really fucking hot lol. If it
cooled or lingered a bit, it would still deprive enclosed pockets of oxygen. But for the open atmosphere it would rise pretty quickly.
I'm not sure if it would disperse
too much to have such an effect.