It doesn't matter if it's plausible or not. It's the fact that it's non-rechargeable device with 12 hours of recording...
Is that worth it to you? Perhaps not. But plenty of us pay similar amounts of money for software subscriptions related to productivity, so it's not especially outlandish compared to paying for Standard Notes, Todoist, etc.
When no one’s looking obviously.
Not sure how that would work with the ring. Either lots of small similar notes. Or a dead battery.
But when you're having a conversation with someone and they ask you to pick up milk from the store later, or you're running to the bus and want to just jot down an idea you had briefly, and other moments where the friction is higher...then this seems like the solution.