@far4 It's true it would have to be written from scratch, but it's not nearly so much a case of ability, as a case of priorities and practicality.
Though small, the Vivaldi development team is one of the more capable and experienced (some building browsers for nearly 30 years) teams in existence. But whether it would be a good idea to support extensions for no less than five mobile platforms and try to maintain this on an ongoing basis while at the same time having to patch Chromium for every release on all of these same platforms is another question altogether - to say nothing of what it might do to mobile browser performance and system drain.
The issue is whether they find it to be a good idea. And so far, they haven't.