I don't have much familiarity with the blog itself but another post says that the property cost $389k[1].
They claim they could've paid cash for it but chose to get a mortgage - so they don't own it "free and clear". Unless they've ended up paying off the mortgage early. Assuming a 25% down payment that's a $310k mortgage. You'd need annual household income of around $80k to qualify for a mortgage that size, which also seems within the reach of a dual-earning household of white-collar/skilled trades professionals.
A household earning $80k annually could save up the $90k down payment for that property in about 3-4 years, assuming a 40% savings rate. I'll admit it's aggressive and ambitious by most people's standards, but not actually impossible.
Now I don't think it's actually a good idea to buy property worth nearly $400k if your annual household income is $80k but it's not an entirely outlandish idea.
I assume the Frugalwoods had annual income well in excess of $80k before they "retired". Their savings rate is also correspondingly huge - 71% according to the article. A brief skim of some other posts suggests they're going to be renting out their current house + putting the new property to revenue-generating uses. They clearly know how balance sheets work and the numbers make sense for their situation.
Pulling back a bit I think we should acknowledge that the Frugalwoods are people who have optimized the heck out of the very good hand that they were dealt. We should be applauding that rather than trying to pick at their story and point out why it doesn't work for everyone. I might not have Michael Phelps' swimming genes but following his workout routine would probably improve my health, even if I never become an Olympian.
It's obvious that having a higher income makes it easier to frugal your way into early financial independence. The vast majority of high-income households don't take that path even though there's not much stopping them.
1. https://www.frugalwoods.com/2016/04/22/the-finances-of-our-c...