Hmm I don't see any mention of equity here but only profit. If you're able to cover the cost of mortgage + HOA/property tax + expenses of running the place then the equity is the profit. Assuming the price of the place is the same in, say, 10 years. Then you've built up 10 years worth of equity buy having someone else pay your mortgage...
Of course if this place was bought cash, you could have done the same thing in reverse. Take that money and loan it to an individual to buy a house. The only difference is there's no extra profit to be made but also no added risk of not finding clients, repairs, etc. (unless someone wrecks the place and it becomes worthless)
I could be wrong but I got the impression he bought it outright and not with a mortgage. But even so, if his mortgage was a traditional 30/yr fixed loan, his monthly payment would ridiculously low. And then he is free to massively over-pay his mortgage payment each month... which is going to pay it off in about the same 4 years he mentions in his write-up.
Of course if this place was bought cash, you could have done the same thing in reverse. Take that money and loan it to an individual to buy a house. The only difference is there's no extra profit to be made but also no added risk of not finding clients, repairs, etc. (unless someone wrecks the place and it becomes worthless)