The article says "Shift logic to the application level to reduce database load."
(Emphasis mine.)
It may have been edited since you commented (perhaps because of your comment), but it seems like they aren't actually advocating to do joins at the application level.
I worked at a company with an extremely complex optimization application with more business rules than any human being could ever comprehend. The logic has to go somewhere. Either the database or the interface or the application itself I guess. There are pro/cons to all. Some of the logic is hard to follow in SQL compared to a general purpose language. The vendor for us of course did some kind of blend where a lot of SQL packages do certain validations and so on. The optimization problem reads a bunch of .csv files from the database and then has its own rules it uses to build the math problem for the mathematical solver.
(Emphasis mine.)
It may have been edited since you commented (perhaps because of your comment), but it seems like they aren't actually advocating to do joins at the application level.