I would think the session-timeout parameter, if set, is a global maximum timer set for the session. Period. The session is only allowed to last 5400s (in your case) and upon reaching this timeout, terminated. I think it's use is more attune to daily limits. i.e. you would set a session-timeout of 8 hours to cover a general business day allowing users to authenticate to the port but not remain connected past the end of the day. Or in a coffee shop you would set it to one hour to allow a transient guest to use the services but be terminated shortly after they leave.
Otherwise re-authentication simply forces the client to re-authenticate, calculate new keys, etc.