Did the Empire have external threats to design against?

Can the Imperial fleet's design be explained by the absence of outside threats?

No; the setting includes external dangers, and preparing for capable invaders would strengthen the case for point defense rather than remove it.

The Empire faces dangers beyond its familiar sphere, including the later Yuuzhan Vong invasion. A fleet intended to face a capable external opponent would need layered defenses against incoming craft and weapons. Those threats make the shortage of point defense harder to justify. Ship design must account for the range of contingencies a state expects its navy to handle.