As far as I’m concerned, if you publish a PRO registered track on Audiojungle, you are your own publisher. This means that you are not only able to collect the writers share of your tracks, but also the publishers. This is of course different from PRO to PRO.
However, there are publishers out there who will offer you to do the publishers job for your audiojungle portfolio, so you can recieve the writers share of your royalties. My advice would be to ignore those messages. They have nothing to offer, since a publisher has to do lots of more work like aquiring clients, negotiating deals, managing their portfolios. Things you and Audiojungle do here.
People who offer you to take over the role of the publisher don’t have anything to offer, expect to get the publishers share, which belongs to you. So please don’t get fooled. Could be that there are credible people who are offering to publish your tracks, but I’d be very wary.