Their mutually exclusive because their a choice between two very strong skills, and imo its a fun choice between spamming damaged ships and making your damaged ships much more tanky. 30% reduced deployment cost is an insane modifier that really does not need any additional effects.
You also dont need the Piracy XO to run Derelict fleets, yes, the effect is very strong, but your argument misses the fact that dmods themself already reduce supply cost a ton through their own 20% reduction to supplies per deployment.
It feels more necessary to go for piracy in addition to improv because improv only has three choosable skills that affect d-mod ships, and to be honest besides the last two they're mostly logistical, which doesn't really help you in a fight. It's more critical to have every advantage when working with d-mod ships because you're already nerfing yourself by stacking d-mods, and as such running d-mod ships is a strategy you need to go all in with otherwise you're just shooting yourself in the foot by using them.
With a five d-mod setup your ships can have around -45% hull and armor if you get unlucky with the ones you get, and that's with most of the d-mod damage reductions already in place. Running fortifications on d-mod ships only really makes the ships *as* durable or slightly more than the baseline ship, which outside of the few advantages you get with daily maintenance is pretty pointless, you'd be better running any other officer that has a bit of logistical in their tree because chances are they'd give you more damage and more durability than running an improv officer.
If improv had more focused skills for d-mod builds, then running derelict operations (The sole skill that makes d-mod ships viable, since you can make up with each ship being around 20% weaker by having 30% more of them) wouldn't be as critical - since you'd be able to make up for it through, say, weapons taking 3% more flux to fire, but deal 5% more damage per d-mod through something like "Overcharged Munitions" totaling a 15% more flux cost, but 25% more damage at max d-mods - allowing you to instead build around making their maluses into benefits like a skill like derelict fortifications suggests.
Another skill that would be a good fit for a fortifications-style D-mod officer would be something that grants a small amount of OP to each ship based on the number of d-mods they have and the ship's size - say 1 op per d-mod for frigates, 2 for destroyers, 3 for cruisers, and 5 for capitals per d-mod to represent having more room for modifications and customizations now that you don't have to care about not interfering with baseline systems - they're broken anyways. "Junkyard Modfications"
Finally, I don't think it'd be completely broken for there to be a skill that added a small amount of hull repair, say .5% a second, with the restoration limit being based on the amount of d-mods you have, say around 25% per d-mod, meaning at 5 d-mods you'd be able to repair 125% of your ship's hull provided the battle lasted long enough (Of course they often don't), but it'd be to represent how familiar the crew must be maintaining and repairing the ship, now to the point where they're able to perform such repairs even while in battle.
Either way though, while derelict operations is a realllllllly good skill, it's the sole reason why d-mod ship builds are viable, because otherwise unless you're running uncapped ship numbers, you're just going to get stomped later game when each of your ships is strictly worse than each of theirs - derelict operations allows you to build up an early fight advantage by outnumbering them, hopefully allowing you to get enough of an advantage that you make up for each of your ships completely sucking and claim a win.
If the d-mod ship officer had more skills that made d-mod ships stronger, and not just cheaper, it'd be easier to pick fortifications over operations.