When there are no tabs (i.e. 2 tabs or more) Thunar does not display /File/Detach Tab; So should be the case for /File/Close Tab (Ctrl + W). Note: By hiding /File/Close Tab I do not mean to disable "Ctrl + W" as an additional way to close a window in addition to "Ctrl + Q".
And thats why its still visible/enabled, if disabled or hidden, the keybinding will stop working too.