The author never said it was a good design decision, or even a desirable one. He only said it would be impossibly difficult to do in a purely functional program, which is worth considering. Whether or not it's a good design decision is irrelevant.
He's making the argument that functional programming is bad. If it were a terrible design decision then the language precluding its implementation (as he claims functional languages do) would be a positive or at least not a negative. So, he must be making the argument that it is a good thing, or at least a necessary thing.
Even so, the ML's all support this type of behaviour (with globals).
In the same way, we do not criticise functional languages for not allowing goto style control flow.
It is very relevant, because author tries to paint this difficulty as a bad thing. It isn't. If the language, or a tool, makes it hard to do bad things and makes it easy to do good things, it's good.
The author never said it was a good design decision, or even a desirable one. He only said it would be impossibly difficult to do in a purely functional program, which is worth considering. Whether or not it's a good design decision is irrelevant.