When RDM is unlocked at lvl 50, or even when its lvl boosted to 90, Acceleration only affects veraero and verthunder. So, to me, its unlikely that a player that is actually reading the buttons of the job they just picked up would assume Acceleration is only tied to Grand Impact. Since the trait adding Grand Impact is only introduced at lvl 96, people that unlock the job normally will have 45 levels to learn the base effect of Acceleration and even job boosters will have 5 levels.

As someone who picked up RDM back in ShB and started maining it in EW, I always just considered Acceleration to be a Verthunder/Veraero specific Swiftcast with the added benefit of guaranteeing a proc.

Part of why Grand Impact gets a highlight is because it replaced Jolt III and Impact, Verthunder and Veraero are always present. If enabling Grand Impact was its initial function and it only gained the instant buffed veaero/verthunder, I'd understand the need more.