Yes, and since I'm also very involved in the OpenAPI Specification, I'm very familiar with this mismatch and the problems it causes. We did try to address that in an appendix to the modern JSON Schema drafts , to show how to bridge that gap, but it hasn't really caught on so far. (apologies for the very long delay in replying, see my "Coming Out of Hibernation" post for an explanation)

My understanding is that an open world allows for indeterminate truth values. Unless there's an error in the schema, it's always possible to determine whether a given instance is valid with respect to a given schema. It's just that the starting point (the empty schema) allows everything. So that would seem to me to be a closed world approach.