These can be, like, pokeballs, but Nintendo version, with Nintendo characters. I can see what they mean by most are invincible. Like, in melee, a lot of pokemon you couldn't attack, if you touched them you'd get hurt, but then there's stuff like Chansey that you can attack and knock away. See, just like Nintendo character pokeballs, basically.
This can also pave the path for future smash fighters in future games, like, some stuff that are just Assists in this game can become full fledged fighters in the next.
Yes, this is one of the best updates yet. These will unlock more possibilities I'm sure of.