If you can, yes... that would work if you had the session uuid, which you might not.
Essentially, when you use SSO to log in a user via a third-party authenticator, NodeBB is maintaining that session, not the other site. That's why it's not assumed behaviour that if you log out of the other site, that you also log out of NodeBB.
If the user wants to log out of the forum, they'd have to log out themselves via the NodeBB logout button.
That said, in the end the logout is literally just a POST request... so in your site, you could code in a XHR that calls POST https://your.nodebb.site/logout and that would log out the user 😄
