I suspect that the app is for forums built on the VBulletin platform. I haven't been here all that long, but also suspect this was once a vbulletin back end.
Another forum I am on switched to a platform that looked a lot like this one. They were able to add support for tapatalk which is now a free app. (I'm sure the plugin was additional money for the forum owners though)
I use Tapatalk all the time with that forum, would be great to see this one accessible via tapatalk as well.