You are correct about WFI piping/welding requirements.
You are also correct that N2 systems aren’t as stringent. N2 can be product contact, since they are used to blanket product But the risk is much less for the following reasons
1 - 100% N2 doesn’t promote microbial growth (while WFI can grow microbes pretty quickly)
2 - N2 typically has a sterilizing filter at point of use (WFI typcially doesn’t have this)
3 - N2 is corrosive on the welds/pipes (WFI is pretty corrosive).
I’ve seen N2 pass with a pressure test and quick weld inspection, but I’ve also seen rigorous standards and treated similar to WFI. It is based on how advanced your company is and thier internal engienering practices.