My guess would be that they went with "ports" instead of "expose" which makes it public. But even with careful composition of your docker-compose you might want to be careful, Docker can interact with iptables in surprising ways and long iptables rulesets are almost comically difficult to validate sometimes. The result is that if you're using both Docker (and even more if you use Compose, Kubernetes, some other orchestrator) and doing anything else with iptables (including using it as a host firewall) you need to take a lot of caution to make sure you don't mess anything up. These tools usually confine their changes to custom chains to make this stuff easier to sort out but at the same time it can make it more difficult to understand what's happening, particularly with the use of tagging and etc.

In general I would recommend putting some kind of protection between machines running Docker (and especially orchestrators) and the internet. This could be cloud provider mechanisms (security groups, ACLs, etc), a firewall appliance, NAT gateway configuration, etc. depending on the situation. It's not necessary, but it makes the situation easier to audit/validate, and more layers of protection seldom hurt. If nothing else it means that much of the time you'll need to make a mistake in two places instead of one, in order to have an unintended exposure.