Hmm, I didn't experience that problem. What version did you test? What region of the world are you in? How are you testing it? Are you testing LAN on a wired network or wireless? Separate devices? Did you investigate routers for port forwarding issues with steam? I tested the project locally in the Midwest of the United States and also on a wired switch using laptops and a desktop. Discovering sessions on a LAN can take a while.
I wonder if your Steam problem has something to do with the test App ID and what region you are located in? They are region locked to my understanding, meaning you have to test it in the same geographical location unless you have your own App ID. On LAN, were you working on WiFi? LAN uses Epic's default online subsystem and I've had problems with it on some networks depending on how they are configured.
The project uses Advanced Sessions Plugin. Maybe your answer is in there?
As far as filtering goes, it's fairly simple how it works but isn't really used by the example project. It looks at the extra variables passed by the online session and removes them accordingly. It might require a bit tweaking depending on how you want to set up your project. (For example, filtering by maps would require more maps than the test project has.)
Thanks for checking out the project. Hopefully you find something useful about it.