The "Who is hiring" threads get initiated every now and again, and then you can post a reply to it. The particular threads you are pointing to are closed to new comments because they are old.

You can start your own "Who is hiring" thread and reply to it with the position you want to advertise, or you can post in the same way as you have here with a job offer, or you can create a web page describing the position, and submit a link to that.