When I first saw the headline, I immediately thought "nope." But then I realized I do know a millionaire next door, and the whole reason they didn't immediately spring to mind is because they live their lives so modestly. My husband's grandfather retired from the military with a full pension, and then went into civil service for another 25 years, so he has two reliable government pensions. In the meantime, they were exceedingly frugal/conservative (even while raising a LOT of children), and have accumulated probably a dozen rental properties. They live in a tiny house in a LCOL area, packed to the rafters with canned goods/clipped coupons/homemade items (ahem, they might also be hoarders). They are in such a comfortable position due to living this way for decades, that when we were in the market for a mortgage, they offered to wire us $125,000 the next day, and said they would match the best interest rate we could find from a bank.* They've done that with several of their grandchildren/great-grandchildren (they have an unreasonably large progeny at this point). I also suspect they may be directly supporting several families, or giving them free rent in one of their properties.
*In general, I'd think such a plan ill-advised, but they are such logical and reasonable people (and, if I say so myself, so are we), that it has been treated like a truly arms-length business transaction. We got a better deal than we could have from a bank (with the added peace of mind that in a true emergency, their foreclosure process would be much more compassionate than a bank's). They got a better return on what their money was currently doing (sitting in a CD). It's been a win-win for over 7 years.