Actually, the Invisibility potion was in the code (I think, I could be mistaken) already along with Night Vision, Jump Boost, Haste, and some others. There was no Item for it (brewing recipe, sprite, and the actual item), so all Jeb had to do was add the item and a brewing recipe and apply the potion code to it.
In other words, yes, it is a potion and not a new mob.
I'm guessing the invisibility potion will be about as useful as it is in Terraria. Which is not useful at all. But I'm looking forward to the other potions. It'll be nice to have some more non-combat potions.
edit: judging by that picture full invisibility would be impossible because even without armor or an item in hand you'd still have the potion effect particles giving you away.