The way I use them is to reflash the firmware with ESPHome, at which point they have nothing to do with Sonoff. I got really into these things when they came out and made a video about the process: https://youtu.be/Kdf6W_Ied4o?si=4nh7kP28IglwVHBx. There are a bunch of different ways to use them including retaining the original software, but I kind of stopped paying attention when I got mine working.
It's worth noting that they have two different products. The "NSPanel Pro" is completely different, I think it runs Android. I haven't played with that at all.
I haven’t seen that software, but it doesn’t look like it can be used with Nextion screens, which are totally proprietary.