NERC doesn't have winterization standards. It has reliability standards that tend to require winterization. Texas would not be in step with the generation standards without more significant winterization, therefore they'd have to improve their systems to meet those standards.

Edit: Here's a PDF of all the standards from NERC. Search "winter" or "ice" or even "cold" and you'll get zero hits.

https://www.nerc.com/pa/Stand/Reliability%20Standards%20Comp...