Yes I saw it the same way. I think the main takeaway is task-switching causes inefficiency so try and reduce that. He offers his way but do something using that idea that works for you.
It makes the assumption that the motivation is already there and he enjoys doing it, only is not seeing immediate feedback.
I think 3 years is kind of arbitrary and not to be taken literally. He's just saying that sometimes these projects take longer than expected to gain any traction
The thing about not being open to other opportunities reminds me of the idea of flow: You don't focus on getting into flow but you focus on avoiding distractions that would prevent you from getting there.
Its different for everyone but I'd say 10% minimum which is still more that the US average.
This is a bit oversimplified but savings rate is the one factor to how long it takes to become financially independent. Most people will want to enjoy life a bit but a high paying thrifty SWE could theoretically save 80% of their income and retire (or be financially independent and work on whatever they want) in about 6 years.
It doesn't work as expected in all countries: https://en.wikipedia.org/wiki/Date_format_by_country