The article seems overly optimistic. 20% cost of goods sounds low to me. You won't get there by selling soda cans at $1 each.
Also, the situation described in the article is perhaps the most favorable scenario in which to own vending machines-- established locations, established business patterns, apparently no electricity costs (this isn't explicitly stated, but no costs are mentioned). Maintenance costs are not discussed beyond stating that these can be reduced by doing your own maintenance. Expected service life of the machines is not discussed, so we are to presume that these machines last essentially forever?
It's funny that the article discusses the owner of these machines as once owning 1000 machines. Someone who owns 1000 machines will be able to give you a range regarding what percentage of revenue will be consumed by maintenance under ordinary operating circumstances.
The end of the article, regarding restocking times surprised me. There seems to be a conflict of interest in wanting to restock the machines less frequently, but wanting to maximize sales.
As mentioned above, it's only really passive income if you hire someone to do the stocking and maintenance for you, the description in the article seems to be more of a self-employed part time job. The income derived from such a job might well be taxed as earned income rather than capital gains (oops!), so even if your COG is 20%, your taxes will be 30%+.
I've run a business. I've never owned a vending machine. When I read an article like the one in the link, I get the impression that the author has never run a business or owned a vending machine. The discussion of expenses suggests little to no understanding of the practical side of the business.