IIRC, SDC does not immediately clear cookies/cache on tab closing. It purges them on a scheduled interval. If you close the tab and wait a while (you should eventually see a toast message, if you have them enabled, saying something like "deleted x cookies from blah.com"), then your session should be removed. Closing the browser runs the purge immediately which is why that was working for you already.
EDIT: apparently it does.
BTW, you can check if you can still be cookie / storage / cache-tracked here: http://www.canyoutrackme.com/