1. No, startups aren't the only way to "significant wealth". In fact, in my experience, startups are something of a lottery, i.e. you only a very small chance of making a lot of money. For every success story you read there will be many more failures. But having said that, it can still be worth founding or working for a startup because you can often learn a much broader set of skills than you can at a big company.

2. Yes, I have heard about a small number of exceptionally highly paid developers too. However, I suspect this is incredibly rare, perhaps with a similar or even lower probability than making your fortune at a startup. Unless you have some extraordinarily talent and are aware of this and in a position to able to exploit this (but I doubt you would be asking the question if this was the case).

3. I would have thought your best chance of making enough money to reach financial independence is the usual unexciting advice: (i) work hard to get an above average salary, (ii) live frugally and save as much as you can, (iii) invest what you save carefully, (iv) continue this process for many years. If there was a sure-fire quick and easy way of getting rich, I'm sure more people would be doing it, or you wouldn't be reading about it here.