Being able to choose between mono and stereo tracks is a DAW-dependent feature. Our plugins show up with a stereo UI when either input or output is stereo (or both, of course). Reaper always uses stereo channels internally, and when they contain mono audio files, they are still always routed to a stereo output. So yes, the fact that the plugin UI shows up as stereo is indeed correct.
In other hosts like ProTools, Cubase and Logic where you can select dedicated mono channels, the mono GUI of the plugin will show up on dedicated mono tracks.
And unfortunately, there is currently no user-selectable option to switch the GUI to either mono or stereo in our plugins as this depends on the DAW's channel input and output configuration.