Well gaaned92 says it’s NSIS, 7zip says it’s NSIS, darktable-packaging.cmake says it’s NSIS…
So I downloaded NSIS to see if I could get some insight (no) and ironically I extracted the NSIS installer with 7zip. 
So yeah, I’m also thinking the dt generated installer script may be the issue. Which I wouldn’t expect to change just to save me the trouble of having to install it. lol