So this is a slightly different situation from the one described in the article. In that article, all sysadmin accounts have been deleted from Windows, and the sa
login is not available.
In this case, your account is available, but it has been disabled.
The remedy would be to create a second account in Windows with administrator rights (if you don't have one already) and then run SQLCMD as this account. (Which you can do with help of the RUNAS command.)