I remembered reading that bookmark export was coming in vers 43. I found the article at:
http://www.ghacks.net/2016/11/07/opera-43-faster-bookmarks-export-and-more/
I guess bookmark export remains in the "Announced features that are not implemented yet" category. bummer.
Seems messed information.
Probably they will implement it soon...
Why didn't implemented yet? Nobody knows... :|