Is it that plist, or is it /Users/xxx/Library/Preferences/loginwindow.plist as per item
~/ (tilde slash) is Unix/OS X shorthand for /Users/xxx so they are one in the same.
Your eyes have led you down the primrose path.
~/Library/Preferences/
com.apple.login
items.plist is an entirely different beast than /Users/xxx/Library/Preferences/login
window.plist.
It's been a long time since we've dealt with an issue involving Login Items, and I'm not sure I was ever certain of the difference between the two, but as I said, I believe the latter governs in this instance.