> If YAML is a strict superset, wouldn't that mean that YAML must have at least one feature that is not part of JSON?

Yes. One of the features YAML supports is the widely documented format we are all familiar with.

However, being a "strict superset of JSON" also means a conformant YAML implementation can load a "pure" JSON resource without issue. The converse is not generally possible as JSON cannot express what YAML can, such as octothorpe ('#') comments.

HTH

EDIT: see also https://news.ycombinator.com/context?id=42361994