It's more than just a GUI for OpenWRT, as it contains a lot of features that OpenWRT doesn't have and over time it'll have more an more unique features compared to OpenWRT.
Some parts are shared, but other parts are unique to NutsOS. It doesn't make sense for us to write an entire OS from scratch, as it's a monumental task to do.
The two work in parallel, but we're not actively working on developing the OpenWRT GUI, it's provided as is. We're of course doing things to the underlying code, but this won't be all that obvious to most users.
Keep in mind that the LCD UI is also interacting with both of the graphical UI's, so technically we have three different UI's talking to the same code base, but they have slightly different feature sets.