The Aegis, the Cruiser and the Athena class are all based on warship program studies that date as far back as the 22nd Century. Very clearly, in universe the idea of putting capital weapons on primitive core JumpShips had been thought about, the rules clearly permit them, naval autocannons predate compact KF drives by a century and had to be mounted on *something*, so the idea that there weren't capital-armed primitive JumpShips prior to the Dreadnought seems silly to me.
We have stats to exactly two primitive core JumpShips: one of the earliest and one common Hegemony era transport, both of which are meant to be somewhat representative of their eras, but hardly the only ones built during those periods.