I find the timeline believable but I’m impressed (and happy!) to see this worked with self-study. I lived abroad for a while and found I was able to pick up the local language pretty quickly because I was fully immersed in it, by 6 mo. I was comfortably conversational for day to day things. However, I would also caution for anyone wanting to learn a language at home, it’s a muscle and it does fade when you don’t use it. After a decade back in the states I’ve lost the majority of the language I learned since there’s no one around to use it with.