The GSW tv commentators made a jokingly reference to it, it was during the huddle before the game. Saying this kind of stuff in those moments is pretty par for the course for sportsmen and it's always been. Not only sportsmen, but in lots of other fields. Fairly common motivational tool. It's not even trash talking.
This has nothing to to with a Celtics attitude or anything. Every other team in the NBA and out of it says similar things. Just a way of players getting pumped up for the game (or troops to a battle, or an executive to a business round, etc).
It's pretty bizarre that nowadays people can have problems with this kind of thing.