Thanks a lot for sharing this information with us ShellShock !
Still, I wonder how you managed to find out that the .html files in the archive were actually gzip files because gzip (on Linux) as well as 7zip (on Windows) are unable to open the ".html" files from the official Kobo dictionaries.
Have you found a way to open the official files ? Or are you aware of any way to do it ?
By the way, thanks to another contribution on this forum I wrote a script that allows converting a dictionary in mobi format to the kobo dictionary format :