It should potentially apply by splash, though I am unsure on the exact splash radius, its not something included in the unit details. That being said it does feel a bit to generic for a faction specific tech.
Currently those who have bolters are Carthage, Athens, Sparta, Ptolomies, Macedonia and of course Rome.
Macedonia is already strong so definitely not them. Ptolomies have machine gun level bolters(Cleopatra boosts fire rate even more) so not them either.
Athens I am unsure where they are at, but they have a bunch of ranged options already. Carthage is very much in the same vein too, albeit weaker. That leaves just Rome and Sparta, two strong infantry civs with very lacklustre ranged capabilities outside of siege units which Rome has a significant advantage on.
Sparta would certainly benefit the most from it given they have no catapults, flaming bolts on their bolters would certainly be an interesting alternative for building breaking.
Rome already has the onagar, but the scorpio is also very iconic to them and they do go hand in hand I would say. They now have a strong theme of manoeuvrability which is cool, so that could be utilised instead. Given their known siege prowess one can definitely still make a case for flaming bolts or something though.