Welp, I finally went through all the dialogue choices with the mammet. It turns out he does mention that you can have more airships as you do more exploration voyages. I guess it was decently hidden but yeah, just read what the mammet says for airships. Easy to miss, but simple to point out.

On a related note, since we have enough parts of the invincible ship, can and how do we change around the parts of our existing ship? :O