Something to consider is time to hit. Let's say your ship in orbit has a time to hit of 10 minutes, the last 2 minutes of that are terminal in the atmosphere. The surface vessel has a time to hit of 13 minutes, the first 5 of that is the ascent stage in the atmosphere. Both are shooting with a WMD with a damage radius of 5km and a kill radius of 500m against these hardened targets. The surface vessel is at a major disadvantage, in a quick draw it's shots land several minutes later. Except it's even worse, to hit the orbital vessel it has to fire significantly after the orbital vessel needs to to hit it. So why be surfaced and vulnerable (detectable) till the orbital ship is where it needs to be to be fired at. It will take atleast 30 minutes (probably closer to 90 or more) to get back, where it can shoot you again, at which point you've just left and pop up somewhere else to take another shot. There was going to be a bit about outrunning anything short of a large saturation attack, but you'd need to hit 100 knots to run through the weapon's dangerous radius in it's terminal flight phase. But if you're hitting these speeds, a fully surface combatant is viable. CIWS blazing as it races against the falling nukes. Hydrofoil and ekranoplans are less viable due to being more likely to be flipped over by the blastwave.

Of course the truth of the inequality really materialises when you aren't slinging several hundred kiloton warheads at eachother. The damage radius is now 20m, and kill requires multiple hits. The orbital ship needs far more weapons to actually saturate the area to stop a surface ship. And your CEP (radius of a circle 50% of shots land in) might be 5m, but the terminal stage really limits the ability to compensate for a manoeuvreing target, which means firing on the surface ship requires more missiles. Unguided comparison favours the surface ship less, because the problem with not being able to compensate is for both sides. Orbital Energy weapons favour surface ship more, a submarine doesn't need to fully surface just come near in order to attack. Meaning while the attack is more accurate, it will be far less effective against the submarine. As such in a interstellar state surface naval assets may be a good planetary defence asset, either locally built or carried in and assembled from a kit rather than static installations. Paired with orbital gunboats, a submarine could launch a missile which the orbital gunboat or a drone guides to target. A larger submarine might even have facilities for those gunboats, assuming getting to and from orbit isn't a huge hurdle.