It's all about your priorities/tradeoffs and how much work you're willing to put into it. They lead a very no-frills budget-restricted life, keep very minimal possessions, put effort into finding overseas teaching jobs in low cost-of-living areas, finding dirt-cheap travel deals, and honestly I think the kids getting all this early travel and cultural exposure is probably a big net win (vs e.g. prioritizing things like buying them the latest video game consoles and fashionable clothing, etc).
You could say the same for starting most small businesses. There are loans available to get up and running if you have a solid business plan and don't have awful/ruined credit, and many viable businesses can be "pay my bills" profitable fairly quickly. You won't make as much excess cash as getting on the corporate treadmill and doing well, at least not earlier on, but you do reap the rewards of being your own boss and learning a suite of skills that's much rarer, and over time even the small business world can eventually become quite lucrative if you want it to be.