This KBA explained the behaviour of corporatePosting and intranetPosting in Job Requisition ODATA API.
SAP SuccessFactors Recruiting Management
The corporatePosting and intranetPosting field under JobRequisition entity is a bool type field. It will show as "true" if the job has been posted and will show as "false" if the posting has been removed. This is same thing as clicking the Post Job and Remove Job button in Job Posting tab. It does not validate if the job posting is expired or not so we may see that it is showing as "true" despite job is no longer posted.
If you are looking for API field that can be used to identify if the job is posted or not, you can use JobRequisitionPosting entity.
2897834 - Job Requisition Posting via OData API
unposted, job requisition posting, _internal, _external, boardId, postStartDate, postEndDateOffset , KBA , LOD-SF-RCM-API , Webservices & APIs , Known Error