Navy is an advanced stat, it is not the same as regular stats. The way AN's clarified it works is that he first compares advanced stats. Whomever has the higher advanced stat than has a crushing advantage when it comes time to compare the primary stat it is compared against. They're worth far more than an equivalent value in the primary stat.
I.e. During the 1st Trelli War, they had 2 Merc companies ~14 Martial and X Navy. We had 2 Merc companies, ~20 Marital, 0 Navy and a crushing tech advantage. That difference in navy, whatever it was, was enough that it allowed the Trelli to turn the campaign into a slog where we eventually gained advantage. If we'd had a level of equivalent Navy, we would've won much faster because we had so much more Martial.
Only because the Pirate King is a naval hero. The update said we were expected to win. The only reason we didn't outright was because the Pirate King managed to save his ships from defeat. Even then, they couldn't contest us over land and thus were doomed to lose. That was why the Pirate King fled in the update before last.
AN also implied that it would be a {S} Influence that hits every subordinate, every turn. That was an insane amount of cultural conversion; we don't need nearly that much. While a Vassal Support policy is not nearly as impactful, it builds up over time and is semi-targeted to the area of most need. It's useless for a crisis, but keeping everyone on the same track culturally is something it can do. It would help a lot with divergent cultures like the Stallion Tribes were.
I'm more interested in it for the +1 Subordinate slot. Depending on what it does, I may change that in a few turns. We don't really have a good idea of what this policy does, so we might as well test it out.
I'd honestly like to see us obtain 4 of these policies. Of course, that has to contend with: 4 Infrastructure (9 progress/turn), 2 Defense, 4 Forest as well.
Our casus belli is going to be quelling disorder in the Trelli Straits. We're acting to eliminate pirates and bandits. Given they are both hostis humnani generius, I'm not exactly torn up by that revelation. We'll kill people, but we're only doing that because they're literally murders, rapists, thieves, deserters, and slavers.
Who the heck is going to push us out? The Khemetri are too far away. The WY are too far away and getting their asses kicked. Free Hill can't take Trelli on its own now, it would be the process of time for them too and the fact that there's no one else to contest them. The Storm Wolves and Metal Workers are both too far away. Are the bandits and pirates that are literally murdering each other and slaughtering Trelli's ravaged infrastructure going to do it? They are going to be starving extremely shortly because all of the farming slaves have escaped in the chaos.
Are we going to lose Trelli to rebellion? I'd like to see them try. 3 Merc companies is 24 Martial. We would have to hit max martial in order to muster an equal number of soldiers! Our entire core! A ravaged and broken city will not be able to manage that. Trelli is also the single best defended position in the entire world. They cannot be put under siege since we control the Black Sea, they're likely to have at least Massive Walls, and they'll have the greatest concentration of military force seen outside of the Battle of Bloody Vale.
The only question is will 3 Merc companies be enough to seize a city wracked by civil war, anarchy, and famine. Virtually all of Trelli's military has gone rogue and is currently pillaging and destroying everything in reach.