I have heard a lot of excitement about ordnance tubes on the Raider, but I think I might be missing something. The card says you can replace you hardpoint upgrade slots with missile or torpedo upgrades. But the great thing about hardpoint upgrades is that you can fire out of them in addition to your primary weapon (sure it costs energy). I have kind of gotten the impression that you could fire ordnance tubes in addition to your primary weapon like hard points, but I don't see why this is the case.

They're still secondary weapons. Hardpoints are a sub-type of secondary weapon.

They're still secondary weapons. Hardpoints are a sub-type of secondary weapon.

Is there some rule that says a huge ship can fire it secondary weapons in addition to its primary weapons rather than instead of.

You can fire your primary and ALL of your secondary if you want when using a Huge Ship in Epic play.

Huge Ship Rules, Page 2

Huge Ship Attacks
During the Combat phase, each huge ship may perform one attack with its
primary weapon and may perform one attack with each of its secondary
weapons. Each attack must be fully resolved before it begins another.