ok so I spent hours thinking about this and I came up with a *hopefully* reasonable theory.

So first, they jump and are faster (when in ink) as squids.

Also they need enough ink to propell themselves forward, and when they get that ink they use their aquatic form for extra speed and height. (Which explains why they are slower when super jumping in enemy ink)

Also if you look closely enough, you can see that the ink comes out of where squids and octopi squirt ink irl. They can't just shoot behind them instead 'cause that wouldn't provide enough power.

So there you have it. My theory as to why they super jump in aquatic form. It's honestly kinda dumb, but I find it interesting nonetheless.