The article "Immutability is not enough" is sub-par in every aspect.

The author never defines, what sort of problems immutable data can help avoid, or what the benefits of immutable data are. And still, after every paragraph, he wonders why functional programming didn't help him avoid bugs in his business logic.

Like, what the hell, the author produced a bug because the order of your operations was wrong? Great, write a test and restructure your code to make it work, at least you know where to look.

Functional Programming does not fix your Game Loop, and it never promised to.