Xanathar's Guild is well known among criminals, but the identity of Xanathar is a closely guarded secret.
Xanathar is detailed in Dungeon #206, in an article titled "The Xanathar: Beholder Crime Lord of Waterdeep". It quotes a thief working in Waterdeep, who says:
I don't know if the Xanathar really exists, but as long as I'm working in Waterdeep, I make sure he gets his cut.
— Termerin, cat burglar
According to this article, many believe Xanathar is fictional, or that if there was ever a real Xanathar he has long since been assassinated or died of old age. The fact that Xanathar is a beholder, let alone that he even really exists, has been a closely guarded secret for more than a century.
The guild itself is highly secretive, to the point that many consider it a rumour; it's described as something "whispered in the shadows of Waterdeep". It's entirely possible for an ordinary person to doubt the guild's existence, and it's possible that they have never even heard of it. Even those involved with criminal enterprises might have only a vague idea of just what the Xanathar's Guild is and who works for them.
The article goes on to say:
One of the keys to the lasting success of the Xanathar is anonymity. The fewer people who know the guild leader is a beholder, the better. Thus it has four lieutenants who carry out its daily operations in Waterdeep: Kal'dir, Quid, Sial Sapphire, and Draak. On occasion they have masqueraded as the Xanathar, their identities magically protected when undertaking the role.