I've been active in my typing practice since I was about 4, and once I'm warmed up I can usually hit high 120s with 100% accuracy on unseen MonkeyType using a qwerty keyboard with red Cherry switches. If I ignore accuracy and go for 'raw' speed, I am able to achieve significantly faster WPM, typically between the range of 140s-180s. The issue I run into with programs like these is that I have to read/match the expected words. Even if I can see the words ahead of time and have a moment to try and memorize, I just go so slow trying to get the correct words. To contrast this, when I write for fun, and know what I want to say ahead of time, I'm fairly certain I'm able to burst into the low 200s (don't have a solid way to test to be sure unfortunately). Depending on the complexity of the error, Grammarly usually spits out correct solutions when I'm around that 75% error rate. The vast majority of the time I'm able to auto-correct everything with a single click. The only exception is when my hands drift and I end up typing something like 'gppnst' instead of 'foobar'. What I mean to get at is that it's somewhat dependent on the types of errors that are made. If it's easily auto-correctable errors at 75%, awesome, the person is hitting an incredible WPM, if instead it needs to be corrected by human thought, then I would completely agree with you.
The real benefit to this speed/accuracy is that when writing long-form text, rather than say programming, the speed of typing can either match or exceed the thought process. There is a huge amount of utility in this approach and the keyboard becomes a true extension of the mind. I'm still waiting to find a good real-time auto-correct that doesn't screw with my flow at high WPM, I'd pay pretty good money to have something that just works out of the box.
On a practical note, I've considered trying to hit the numbers that the author claims, but I'm already so limited by my own train of thought. With the additional finger/wrist strain (And yes, there is _significant_ strain when these levels are approached for any length of time) I just don't see the costs making sense when everything can be fixed in just a few clicks after typing a great many paragraphs of text.