Hmm... this is just plain weird...
The checkbox toggles an option, simply called "twitter", which - rightfully as you say the checkbox is unchecked - is not present in your database array.
If the "twitter" option is not present, the whole twitter class file which creates the tags will not be included in the script, so it could never generate the tags...
The only thing I can currently think of as a possible cause is if - and I say *if* - the twitter option has moved between two of the wpseo options somewhere in it's lifetime and there is a residential twitter option in one of the other wpseo options...
Could you have a look for it ? I won't need to see the contents of the other wpseo option, I'd only need to know the name if you *do* find it somewhere.