Thanks so much for your response. I received the following email from IBM after opening a ticket with them.
Unfortunately, Domino does not currently implement a SAML 2.0 single logout feature. The logout of a session is configured in the identity provider side and it is configured in the idp catalog database.
Configuring the Single logout URL means that users are logged out of the identity provider side when they log out of IBM Domino
This is a known SAML configuration limitation that is planned to be addressed in an upcoming release but there is no exact timeline for it.
I have subscribed this PMR to SPR # XHXH994598 - Enhancement request for Domino to have a logout feature when using SAML authentication.