I think I found the source for that. Doesn't say whether Glass revealed his identity, though.
https://www.theguardian.com/education/2001/nov/24/arts.highe...
> Einstein on the Beach was premiered in Avignon on July 25 1976. Glass and Wilson were then offered the option of two performances at the Metropolitan Opera in New York, where the critical reaction was delirious: "One listens to the music just as one watches Wilson's shifting tableaus," wrote John Rockwell in the New York Times, "and somehow, without knowing it, one crosses the line from being puzzled or irritated to being absolutely bewitched." The day after the performance, Glass was back driving his taxi: "I vividly remember the moment, shortly after the Met adventure," he says, "when a well-dressed woman got into my cab. After noting the name of the driver, she leaned forward and said: 'Young man, do you realise you have the same name as a very famous composer'."