If raspberry pi doesn't have NAND, but libreelec works.
Why do you compare to RPi? It is designed to boot from SD card. Your box is not.
If the twrp runs with the uboot into sdcard in bricked nexbox.
If you can run TWRP, you should also be able to flash stock ROM. Please don't ask me how, I cannot guide you.
Why does not libreelec install in nexbox without NAND?
Because it is not designed to do so. Every box needs a different bootloader and I'd have to create 100s of builds only because of this. My builds require a working bootloader (u-boot) in internal memory.