Swagger 2 was either YAML or JSON. Since YAML is an evolving standard, OpenAPI 3 just specifies that your YAML should be in YAML 1.2.
Likely a change that won't affect anyone; it's just a clarification.
I see, maybe it should say 'specifies' and/or whatever other edits needed to make that read like English.
By declaring conformance to YAML 1.2 and requiring conformance to the JSON Schema ruleset defined in YAML 1.2 we can ensure that any YAML OpenAPI document can be converted into JSON without any loss of information.