Thanks for the report and sorry for the late response. Unfortunately there is no easy fix for this. But I've updated the addon description to acknowledge this limitation.

You've probably figured this out already, but just in case: You can click on the last visible Video (Number 200), then the export will give you Video 1-400.

If you have any other issues, please report them at https://github.com/Zauberfisch/firefox-youtube-links-export/issues (because I don't usually read reviews here)