I’m confused, the blog post backs up what you say but I can still set custom ports within my account page… And I’m currently running a service that needs to advertise out on a port to work from Mullvad.
Try creating a new torrent with some random file, seeding it from a Mullvad device and downloading it from a different Mullvad device. That should only work if you have port forwarding set up (or if you're not actually going through Mullvad - you will see that by the peer IP in the torrent client).