In the NTT-Assemble.exe there is an option:
4: Toggle Steam
What it does?
It says that "Steam features are now enabled". At first I thought it has something to do with achievements (as I don't get steam achievents with NTT normally) but after "toggling steam" in NTT I don't see any differences with the NTT without "steam toggled" as I still don't earn achievents.