Since the whole idea of ‘opt in’ is choosing to do something, the only reasonable interpretation of ‘opt in by default’ is that the default state requires you to make the choice yourself. However I try to bend my brain, sing the term ‘opt in’ for the exact opposite makes no sense whatsoever.

Note that I’m not using English grammar as my argument; human language (and English especially) has plenty of exceptions and expressions which make little sense, and if people were to adopt this phrasing en masse, it would naturally become a part of the language, whether it makes sense or not. My argument is that adopting this phrasing is a terrible, user-hostile decision and in my honest opinion we really, really, really ought to avoid it.