You mixed up Compose (the config file format) with Docker-Compose (the cli tool)
The link refers to Compose as in the compose file format!
You first sentence about docker-compose is still valid: it is a single host multi container orchestrator
When it commes to swarm deployments, you will have to create a swarm cluster for this (docker swarm init, docker swarm join) and deploy the docker-compose.yml configuration with docker stack deploy.
They simply used the same file format and adapted it for swarm (v3 reference), while keeping compatibility with docker-compose.