SOLVED !
There never was such option. Opera Chromium used to import settings from Opera Presto automatically, don't know if it's still the case.
You were completely wrong! I tried installing older versions of the new Opera and it turns out that most of them support importing Opera 12.x profile - from within the "Import bookmarks and settings" as well as upon the first launch.
Here's the proof:
Screenshot
So, what I eventually did, was installing Opera_41.0.2353.69 (which imports Opera 12.x profile automatically), and then perform auto-update to the current stable version. All passwords and all bookmarks got imported. Brilliant!