let page = document.getElementById('buttonDiv');
duplication: As a result of duplication, they somehow ended with this line being present here but not in the downloadable file which is supposed to have the same contents as the snippet shown here.
They should have updated their static page generation script/process so that their main document referenced options.js and their generator literally included the contents of the referenced file inline. Then there is no chance of the contents of the inline snippet and downloaded snippet diverging and not matching.
Evidently, however, the contents of each are managed independently (in other words, there is duplication), easily leading to the contents diverging, as they did.