It's a bit arbitrary to have it just affect sparkshot, since the other two are both "abilities that deal damage," but I agree with you in terms of the spirit of the effects:
Anti-air and Overpower both have to do with directing damage that is already "there" in the form of a units ATK stat. Sparkshot creates new additional damage (albeit still combat damage), which seems to be what Hotter Fire is all about boosting. Also, it's literally called "spark shot" which means a shot made out of a bit of fire, so it makes sense that Hotter Fire would mean hotter sparks.
Also, anything that makes Jaina better is worthwhile in my book.