twirly you are correct...
if your password was FRED on tuesday and they logged in as you
and you reset your password to MARY on wednesday... but they remain logged in...
then until they close their browser their session cookie will remain active as it was authenticated against your FRED password on tuesday when it was correct...
so they could still be logged in having used an old password...
two basic fixes for that - store the encrypted password in a cookie on the user's computer and check that every page (slows server down, but more secure) once you change your password - next check would fail...
terminate the session cookies from the server - not sure of detail of MN setup or how, but that is possible and in theory is what they were doing when they said they would force everyone to log out - by killing the session cookie from their end your session cookie would no longer be valid and you would need to log in again...
if you were able to go to an already logged in computer after that apparently happened, then the termination at the server could not have worked as intended...
however it is unlikely that the hacker has really got 3,038 tabs open :) trying to maintain sessions...