C# is a language where functional and imperative are freely intermixed, most notably through the LINQ library that shows up in some of the author's C# code examples. The purpose in doing so is not to constrain side effects. It's to make code more declarative and enable the use of compositional patterns.

I think that's where the article really comes across as tone deaf. If you're going to write an article with a title like, "Mostly functional programming does not work," then you better be prepared to demonstrate that mostly functional programming fails accomplish the thing it sets out to do. Instead, what he really demonstrates is that he fails to understand that there are desirable things about functional programming aside from purity and laziness. Which is in itself a rather fascinating thing to miss considering that the majority of functional languages are neither lazy nor pure.