No it's because when you closed your laptop, you put your computer to sleep. When your computer is put to sleep, the connection to IB is still maintained and not broken so you are not logged out and that's why you have access to IB's TWS immediately once you open up your computer and that's also why you can't log in again. IB only allows one session under the same name at a time.
You are right and thank you for setting that straight. I am just wondering then why I received the message the second time I opened the IB app after having opened the app earlier on my walk: "Another session with the same user name already exists." At this point the latest login was through the IB app on my phone an hour earlier, so it's not "another session." Maybe because I had moved to a different location and thus connected through a different mobile mast/base station?
Regardless where you connect from, as long as you are still connected to IB, IB will count that as a "session" and won't allow another to open.
I asked support this very question about a year ago or so. Back then this did not exist, but I created a feature request for it. The suggestion of support back then was to call them and they might be able to figure it out on the phone. Not sure if this feature has been created in the meantime, I haven't seen it.