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.
Now granted, he did not say that, and I only reached that conclusion by the rest of this conversation, but I think to have meant that and said '... was now also opt-in by default...' isn't unreasonable.
Just sharing since you mentioned you couldn't see how using that term for the opposite would make sense.
No, it's really not. It should read "opted in to training on prompts by default".
"Opt in" means "you have to affirmatively turn this thing on". "Opted in" means "the thing is currently enabled".
Which is why it's probably better to write it as "enabled by default". Much harder to misinterpret.