I believe the GP is just cautioning rando HN readers that they should not rush out and make their down payment in the manner described, as opposed to liquidating some of their stock options for "real cash" like the GGP had to do.
They are just explaining a reasonable method that the (above) average HN reader could use to be in the same situation as Bezos of having a 0% tax on their down payment.
In the US, there's a pretty massive exemption (well, deferral) for capital gains tax on the sale of a primary residence, so once you have one home to work with, the down payment is (kind of?) tax-free anyway.
Never give absolute financial advice to anyone who's situation you don't fully understand.
It's not uncommon when people buy deals while traveling or in hot markets.
See also Mr Money Mustache's articles on this topic. He assuredly is not Bezosesque.
Another very rational reason for such a margin loan for a home down payment is if the stock you wanted to sell hadn't been held for a year and therefore its sale would not yet qualify for long-term capital gains rates.
You might choose to pay margin interest for up to a year so that the stock sales become taxed at the much lower long-term capital gains rates instead of like income.
That might make sense for someone in the 24% federal bracket which ends at just under $200K of annual income, depending upon how much longer one needs to hold the position to achieve the more favorable taxation. Certainly far below the yacht-owning bracket.
It's not particularly hard. Just have enough collateral to not get margin called. And, like the margin interest rate better than the tax hit. Shop around for rates. Notice, you don't have to pay the entire down payment this way.
If you have amassed 6 figures of stock and are buying a house, you're qualified to educate yourself on these topics. It's usually worth reading up anytime you incur that sizable a taxable event.
I am not saying this is a great idea, BTW. Just, it's an idea within many people's reach.