You must complete the form truthfully. At the time you're completing the form — now — you are not employed because your postdoc has not yet begun. Therefore, the correct response is "not employed."
If you think the examiner may wonder why an unemployed person is seeking a J1 visa, then add that you couldn't find a job after graduation because of Covid19, but have now been offered a postdoc. That statement is also true.
Whether this will affect the examiner's decision is only knowable by filing the application to see if it is granted. In any event, however, you must state only true things in your application.