At a guess, Container Service was built in order to build this. I.e. Lambda is probably an ECS cluster creating and running Node.js-based Docker containers. (A bit like Heroku's original Ruby stack, actually.)

It's a bit amusing to note that they could have built Lambda on top of their own previous Elastic Beanstalk Docker offering instead, but chose to build ECS presumably for cost reasons (which everyone who has evaluated Elastic Beanstalk's Docker support will understand.)