There's more nuance to this.
Prior to now, the original ARR primals were Invulnerable (it was unmarked, but they were) while charging and briefly after using their 2hrs. Anything you did to them during that time wouldn't count, including trying to build stacks or maintain BLM element. This change makes it so that you can still debuff them, build stacks, etc, even though they're not taking any damage.
Invulnerable means nothing happens at all when you hit them. Invincible means you can still hit them, but they take 0 damage.