Why the plasma lance is compared with Rome's corvus

Is the Covenant plasma lance's limitation comparable to arming a ship solely with carronades?

No. The preferred comparison is Rome's corvus, whose effectiveness depended on an opponent coming close enough to be held and boarded.

The corvus comparison concerns the conditions needed to exploit a powerful weapon. The Roman boarding ramp held an enemy vessel while soldiers crossed, so the opponent had to come within reach and be unable to pull away. In the fictional comparison, the plasma lance similarly works best when the enemy accepts the engagement its users expect. Its long range can also influence how opposing ships approach, giving it a role in shaping the battle.