As well, it's important to note that the use of ballistae wasn't sustainable in Marth's time. It's stated in Kris's conversations with Beck that the oil needed to operate a ballista was in incredibly short supply, and it was currently all taken and used by the Archanean Empire. That's actually the in-universe reason as to why there are no player-controlled ballista units in FE3/FE12, and why Beck and Jake got reclassed in New Mystery.
So it's kind of implied that they started using ballistae much earlier than they normally should've, since using them relied on a magic fruit's oil instead of, well, actually building a ballista that functions.