I’ve been having the exact same problem, products showing the less significant design by default in the shop. Unfortunately, it looks like you need to select the main design when you first set up the multiple print area product (upload or combining existing designs). As it says in the article you linked to, “Please note that the new feature is not yet available for existing products with multiple print areas.”
It’s kind of disappointing that Spreadshirt still hasn’t extended the feature to existing products (as you point out, the article was published in November 2019), but it looks like the only solution for now is creating the product(s) from scratch.