Thanks rjp123, this has come up from time to time.
I think I know where the error crept in.
In some of the old employee guides in the same place (e.g. 2009), it stated:
"UA and US SAG customers may only access the Red Carpet Clubs and US Airways Clubs when traveling in conjunction with international travel."
Since US is no longer with them, they just took out US and voila
"UA and SAG customers may only access the United Club when traveling in conjunction with international travel."