Yes, she probably thought she was playing, but you were right to make her stop.My dog is doing a bit better with the chickens...Peach was holding her down with her paws and ruffling her feathers(?). Idk what she was doing. She probably thought she was playing with the Esper the pullet.
It is easy for a dog to play too roughly, and hurt or kill the chicken.
I think most chicken-killing dogs are doing it for fun ("play"), but the intent doesn't matter to the chicken!