Well, Assault is highlighted when he's doing the aerial attack as well, so I'm not sure how it will tie in with magic exactly, it's one of those things where we don't really have a lot of information. I don't believe he's actually holding it down though, but perhaps he might be like how comboing can also be activated by holding the assault button down.
Perhaps L1 functions now as a Magic modifier which functions similarly to a shortcut menu from KH, but instead of being based on L1 and one of the face buttons, it would be L1 and a direction and the assault button. Think of how the direction works with combos in Duscae 2.0. A normal sword style weapon holding back while attacking does a back flip off the enemy, you can also side step with left and right, etc. Imagine if you can bind abilities to the directional button on the magic button and cast them with the assault button. Rapid fire for short bursts for the spell, or holding the button down could channel a bigger spell.
The problem I have with this approach would be that you couldn't move and cast at the same time, so perhaps instead of the directional input altering the magic spell, you would hold down L1 and the weapon switcher will change to spell switcher, and now while holding L1 and using assault you can cast spells and similarly hold the assault button down to cast a bigger spell. Then, to change spells you would use the d-pad instead of directional input and you can just move freely.
Of course, we so no reference to that in the UI... so honestly I have no clue.