BTT shows a conflict with AltTab even when AltTab isn't running.
And the shortcut (opt + tab) has been deleted in Alt+Tab 
I believe this is the BTT's conflict menu, meaning you have 2 shortcuts opt+tab configured in BTT it has nothing to do with AltTab.app .
Oops, you're right 
I actually have a shortcut in BTT with that name and had forgotten about it... A quick look at “Recently Used” would have been enough. Thanks @xidiot