Okta is a large, enterprise identity provider. I doubt in the case of a right-wing site existing they'd give out access to that provider on purpose? Did they mess up? I can't find any details about it yet.
It's entirely possible that Parler stored all of their data in S3, with metadata documents, and someone managed to find some creds in the app to list the object storage. These practices and this kind of vulnerability are both common.