The problem being that when you compose two functions the number of possible input combinations to function 1 multiplied by the number of possible input combinations to function 2.
When you have 10 functions, each with two inputs, combining them gives you 2^10 combination to test - 1024 combinations.
Whereas if you test each one separately you can test 20 combinations, and get the same coverage.
You might be thinking more of testing several expressions independently, e.g. assert([f(x), g(x)] == […]). This does what you’re talking about, but I don’t think anybody (including the author of the post) was recommending this.