I see it now. The x-data-type appears only for entities, and is not Swagger-related, but specific for XData.
Actually, JSON Schema doesn't have a "int64" (or "long") data type. All types are integer, and only the "format" can be set as "int64", but still that is not displayed in SwaggerUI.
We have improved our Swagger generation here to include the "int64" format, and next version will include this improvement, but that won't change the UI for the end-user, it will still show the type as integer.
Nevertheless, note that what is displayed in the JSON specification. It's not the data type of the underlying class in the development language (in this case, Delphi). In JSON, integer also includes Int64 types. There isn't much that can be done here.