As an MBA teaching tool, it's fine. Students are supposed to read all kinds of crap, so they can learn how to tear it apart.
But as good advice, that article falls far short. I'm not even an MBA and I can see a handful of obvious problems.
1. Please tell me where I can find this "risk free, conservative" tax free 8% investment they keep suggesting as an alternative place to put your money. It doesn't take a genius to see that a guaranteed 8% return is better than avoiding a 5% interest rate.
2. Their tax treatment is bogus. You don't actually get the full benefit of the mortgage interest tax deduction. Hardly anyone does, because you would have taken the standard deduction anyway. Mortgage tax deduction only has any value to you if you itemize, and can deduct more than the standard deduction, and then it is only worth the amount by which your deduction exceeds the standard deduction. For poorer people with smaller mortgages, this is probably zero.
3. The emphasis on liquidity is presented in a vacuum, as if your only options are mortgage debt plus savings or equity plus cashflow problems. What if you bought a reasonable house and could float the payments regardless of equity paydown? What if you have a million dollars in your taxable account and are using the mortgage debt like an appreciable bond, in which case you WANT a low fixed rate? What if you live in San Francisco, where the average time on market is 11 days? What if you have a Heloc? Liquidity is a significantly more complicated topic than they have grappled with here.
4. Did they totally ignore capital appreciation/loss? Houses that cost 85k and houses that cost 850k do not experience identical price swings or carrying costs. In my area, the rate at which your mortgage gets paid down or invested is inconsequential compared to 40% leveraged appreciation every year.
5. The overall message here seems to be "rich people get richer because they take more risks." No shit, Sherlock. Rich people never worry about having enough to eat, or a paying a surprise dental or car repair bill. They get to play with their play money while working stiffs try to make ends meet. Anyone can get rich with leverage when times are good, but only the rich can afford to risk the times being not so good. The rest of the world needs a more conservative fall back plan to avoid total wipeout, and that prevents them from benefiting from high risk high reward strategies like this.