My understanding of the situation is that while ERCOT oversees and coordinates the various utility companies, they don't have the authority to order certain things, such as winterizing production facilities - they make recommendations (and have made winterization recommendations since similar occurrences in 1988 and 2011) but can't force the utility companies to carry them out.
If that is indeed correct, then it would be the individual utilities that would be responsible, though the legislature might wish to give ERCOT more authority in some areas.
Mike