I therefore think it's more interesting to break it down by how you get that time. I see a few categories:
1. Have rich parents. Yes, yes, this is true of some people (but probably fewer than you think), and isn't very interesting except for starting political arguments.
2. Have a supportive partner on whose earnings you can live. This is much more achievable for many people, and under-emphasised (it's common for, eg, postgraduate education!)
3. Have an existing consulting work that gives you steady work 1-2 days per week. People say "startups die by consulting", and they're right, but what kills you is finding new work (it becomes the "top idea in your brain", which is lethal). If you have an established client who's happy to spend a year paying for 1-2 days a week, you're perfectly positioned. (This is how we started https://anvil.works, and I can't speak highly enough of it)
4. "Productised consulting" - consult full time until you come up with a product that all your clients will buy immediately, then switch to selling that product. This sounds great in theory, but I've seen a few people try and fail to make that leap (possibly because it's really hard to stop "find new consulting work" from being top idea on your mind, especially if you have employees/partners to feed).
5. Have a business with good cash-flow characteristics that can be launched fast enough to become self-sustaining within its first month. This is what this article seems to focus on, but apart from "productised consulting" (where you start out selling something that's not your product), I'm very skeptical that it really works for product businesses. If you try this, I'd recommend having a fallback to one of options 1-4!