I'm in a business that requires a fair amount of capital to start and it's funny to me how I ended up using a slow-burn long-time method that is very much like a mini-FIRE game in itself.
Saving, slowly purchasing equipment needed, building a network and client lists and nurturing relationships. 10 years later, and I have a company with equipment, relationships, clients, cash in the bank and credit lines available. Not the best approach for every industry or company, but for me, it really worked well since my business is also a passion and something I love to do. I think this method can work well for people who are trying to parlay a career into a business. I'm not sure if life is long enough to do this and reach FIRE at an impressively young age, but some businesses can move pretty quick, and it depends on what's important to you. What's enviable about MMM's position is that having hit FI very young he has more flexibility to try anything. If my business goes down, it's back to the drawing board. But, conversely, having no artificial cap on my income potential is a freedom I value, as well as the opportunity to employ friends and people I love working with.
I also have a friend who started a business requiring a large amount of working capital up-front and the route he took was to use his passion, excitement & energy, and frankly mind-blowing knowledge on his area of expertise to make a pitch to real investors for his product.
It worked for him, but it took a sizeable effort (mountain-moving, really) to get to the point where money was wired into their company's account, and now he essentially has a boss, but there really is no better option for some ideas.